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 EUG. DUBOIS 
 

THE CLIMATES 
 
 OF THE 
 
 GEOLOGICAL PAST 
 
 AND 
 
 THEIR RELATION TO 
 
 THE 
 
 EVOLUTION OF THE SUN 
 
 EUG. DUBOIS 
 
 LONDON 
 
 SWAN SONNENSCHEIN & CO. 
 PATERNOSTER SQUARE 
 
 1895 
 
PRINTED AT NIMEOUEN (HOLLAND 
 . C. A. THIEME OF NIMEOUEN (HOLLAND) 
 AND 
 
 TALBOT HOUSE, ARUNDEL STREET, 
 LONDON, W.C. 
 
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 PREFACE. 
 
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 The present essay is an attempt to explain by 
 changes of the solar heat the great climatic changes 
 of the Geological Past. It is a translation, with 
 some small alterations, of a treatise in German : 
 “Die Klimate der geologischen Vergangenheit und 
 ihre Beziehung zur Entwickelungsgeschichte der 
 Sonne”, that appeared in the beginning of 1893 
 (Nijmegen, H. C. A. Thieme), — a new and 
 enlarged edition of an essay in Dutch, which appeared 
 in the beginning of 1891, under the title: “De 
 klimaten der voorwereld en de geschiedenis der zon ” 
 in: “Natuurkundig Tijdschrift voor Nederlandsch- 
 Indie”. Deel 51, p. 37 — 92, at Batavia. 
 
 I take this opportunity of expressing my gratitude 
 to my friend Mr. Th. Delprat for his kind assistance 
 with the translation of the German text. 
 
 Both this text and the translation I have revised. 
 
 I hope this short essay to explain the great 
 enigma of the climates of past ages may contribute 
 towards its solution. 
 
 EUG. DUBOIS. 
 
 Toeloeng-Agoeng (Java), 
 
GEOLOGY LIBRARY 
 
 Return this book on or before the 
 Latest Date stamped below. 
 
 Theft, mutilation, and underlining of books 
 are reasons for disciplinary action and may 
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 University of Illinois Library 
 
THE CLIMATES 
 
 OF THE 
 
 GEOLOGICAL PAST 
 
Digitized by the Internet Archive 
 in 2017 with funding from 
 
 University of Illinois Urbana-Champaign Alternates 
 
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 https://archive.org/details/climatesofgeologOOdubo 
 
CONTENTS. 
 
 Preface 
 
 Page 
 
 V. 
 
 PART I. 
 
 The Climates of Past Ages according to Palaeonto- 
 logical and Geological Data. 
 
 The Organisms of Past Ages as Witnesses of Climatic 
 
 Changes i 
 
 The Fossil Arctic Florae as evidence of Warmer Climates, io 
 
 Climatic Zones of former Periods 23 
 
 Local Anomalies in Climatic Conditions 33 
 
 The Hypothesis of a Tertiary Displacement of the Poles 
 unnecessary and insufficient to explain the Phenomena. 45 
 Gradual Tertiary Cooling of the Climates and Pleistocene 
 Glacial Epochs 55 
 
 PART II. 
 
 Changes in the Solar Heat as the Agency by 
 which the Geological Climatic Changes 
 were brought about. 
 
 Considerable Changes of the Solar Radiation sufficient to 
 
 account for the Climatic Changes 72 
 
 The postulated Changes of Solar Radiation have actually 
 taken place 99 
 
VIII 
 
 CONTENTS. 
 
 Page 
 
 The Solar Heat arises most probably from Contraction. 
 Seeming Inconsistency with Geological and Biological 
 
 Data 132 
 
 Further Circumstances in favour of the View that the 
 Geological Changes of Climate are to be explained by 
 the History of the Sun’s Evolution 156 
 
PART I. 
 
 The Climates of Past Ages according 
 to Palaeontological and 
 Geological Data. 
 
 The organisms of past ages as witnesses of climatic changes. 
 — The fossil arctic florae as evidence of warmer climates.- — 
 Climatic zones of former periods. — Local anomalies in climatic 
 conditions. — The hypothesis of a Tertiary displacement of 
 the poles unnecessary and insufficient to explain the phe- 
 nomena. — Gradual Tertiary cooling of the climates and Pleis- 
 tocene Glacial epochs. 
 
 The Organisms of Past Ages as Witnesses of 
 Climatic Changes. 
 
 It has long been an established fact 
 that the climates of past ages were differ- 
 ent from the present. It is proved that 
 in many regions of the globe during untold 
 
 i 
 
2 
 
 THE CLIMATES 
 
 millions of years a warmer climate prevailed 
 than that of the present time, and that 
 this afterwards gave place to a cooler 
 climate than the present. Nevertheless, 
 during some periods a general regularity 
 in the diffusion of heat over the earth, 
 resembling the present, is not to be mis- 
 taken. 
 
 We judge of the climates of past ages 
 chiefly by the fossil remains of plants and 
 animals. As a rule the organisms of warm 
 countries differ greatly from those of cold 
 regions, and, though not with indisputable 
 logic, we infer inversely that the climate of 
 a former period must have been warm or 
 cold according as its organisms are most 
 nearly related to those now existing in warm 
 or in cold regions. This inference is cer- 
 tainly quite justifiable when the relationship is 
 very close, and is apparent in a great many 
 species of plants and animals. In many 
 
OF THE GEOLOGICAL PAST. 
 
 3 
 
 cases we may also infer considerable 
 changes in the climate and its temperature 
 from the greater or lesser energy which 
 life in general displays. Now, we know 
 for certain this general close relationship 
 between fossil forms and organisms which 
 exist at present in different climates, and 
 also that many periods developed quite a 
 different wealth of plant and animal life 
 from the present flora and fauna. 
 
 Yet a fixed relation between the climate 
 and the character of the organic world 
 does not exist. Closely related plants and 
 animals are met with in very different 
 climates, and this was also the case in 
 past ages. Elephants and rhinoceroses, at 
 present true inhabitants of the tropical 
 regions, existed in Europe during the 
 Glacial epoch. Comparing parts of differ- 
 ent continents situated in the same climatic 
 zone with each other, we see that a nearly 
 
4 
 
 THE CLIMATES 
 
 perfect similarity of climate does not cor- 
 respond at all with a similarity of organic 
 forms. Further, plants and animals are, 
 in general, certainly most fitted for the 
 climate in which they exist, but it is 
 seldom known to what extent their existence 
 depends upon conditions which have no 
 causal relation with the climate. Instances 
 are well known of European plants and 
 animals, which, under the most different 
 climatic conditions, have spread over the 
 whole globe, and have even caused the 
 extermination of indigenous species in their 
 new habitat. 
 
 Organic beings have, indeed, in a high 
 degree the aptitude for accommodating 
 themselves to the temperature of the cli- 
 mate. In this respect the late eminent 
 geologist M. Neumayr, who contributed 
 much to the right appreciation of the data 
 according to which we judge the geological 
 
OF THE GEOLOGICAL PAST. 
 
 5 
 
 climates, reminds us of a cause of error 
 formerly pointed out by Starkie Gardner, 
 de Saporta, and others. Organic forms 
 originating in a cooler climate may gradu- 
 ally spread southerly (in the northern 
 hemisphere) whilst accommodating them- 
 selves to the new conditions. They accli- 
 matise themselves to these new and on 
 the whole easier conditions, and degenerate. 
 In the meantime, new forms will have 
 developed in their former home, which in 
 their turn will pursue the same course and 
 suffer the same fate. In this way southern 
 types will always display a certain relation- 
 ship to the older forms of the northern 
 regions, the cause of which is thus not 
 to be found in a change of temperature 
 in their respective stations. 
 
 As early as 1856 Hooker drew atten- 
 tion to the fact that in a uniformly warm 
 and damp sea climate, luxuriant vege- 
 
6 
 
 THE CLIMATES 
 
 table and animal life may exist at a lower 
 mean temperature than in a dry conti- 
 nental climate. This accounts for the fact 
 that the forests of New Zealand and Pata- 
 gonia bear a subtropical character, the 
 temperature yearly not being higher than 
 that of France and England.* 
 
 From all this it will appear how cau- 
 tiously we must proceed when attempting to 
 draw any inference as to the temperature 
 of a former age from the data of palae- 
 ontology, f When used cautiously, however, 
 these data are of great value, the greater 
 the more recent the period is whence they 
 
 * Compare: M. Neumayr, Erdgeschichte. Vol. II. Leipzig 
 1887, and his lecture: “Die klimatischen Verhaltnisse der 
 Vorzeit.” Schriften des Vereins zur Verbreitung naturwiss. 
 Kenntnisse. Wien, 1889. Under the title of “ The Climates 
 of Past Ages” a translation of this lecture appeared in Nature, 
 Vol. 42, 1890, pp. 148 and 175. 
 
 ■fSee A. Woeikof, “Die Klimate der Erde.” Vol. I. 
 Chapter 12, Jena, 1887. 
 
OF THE GEOLOGICAL PAST. 
 
 7 
 
 are obtained, as no important changes in 
 the organisms can then have taken place. 
 When we find in Central and Northern 
 Europe, in a formation as recent as the 
 Pleistocene, remains of the arctic willow 
 (Salix polaris ), the dwarf birch (Betulanana) , 
 of Dry as odopetala , Polygonum viviparum, 
 and Ardostaphylos uva-ursi y together with 
 the remains of true arctic mammals — the 
 reindeer ( Ce?vus tarandus), the glutton 
 ( Gulo borealis ), the arctic fox ( Canis lago- 
 pus), the lemming Myodes torquatus and 
 obensis ) — and in the valleys the chamois 
 and the ibex and also arctic and alpine 
 snails ; in the diluvial marine deposits such 
 mollusks as Peden islandicus, Yoldia ardica , 
 Astarte borealis and Saxicava ardica — then 
 we may safely conclude that in the Pleis- 
 tocene period the climate of Europe was 
 cooler than it is at present. When we 
 furthermore find in the Middle Ter- 
 
8 
 
 THE CLIMATES 
 
 tiaiy formation in France and Switzer- 
 land — that is, in a comparatively recent 
 geological period — palms, fig-trees and 
 cinnamon-trees, together with other plants 
 with evergreen coriaceous leaves, tulip- 
 trees, ( Liriodendron) , camphor- trees, laurels 
 and myrtles, like those which grow in our 
 days in countries with at least a sub- 
 tropical climate, and together with many 
 other species — the remains of the gigantic 
 Dmotherium and Helladotherium of Mastodon 
 and Rhinoceros, together with anthropoid 
 apes ; also trogons, parrots, pelicans, ibis- 
 birds and salanganes, — in the ancient sea 
 bottoms many species of mollusks of the 
 genera Cypraea , Oliva, Mitra, Conus and 
 Terebra, and near Vienna reef-building 
 corals of species still extant, — then from 
 these numerous facts we may safely con- 
 clude that also in this much more remote 
 period an entirely different, warmer climate 
 
OF THE GEOLOGICAL PAST. 
 
 9 
 
 prevailed in Europe. Not taking into 
 account the power of acclimatisation which 
 species and genera undoubtedly possess, 
 and the influence of the insular climate 
 that prevailed in Europe then, we may, 
 with Heer, conclude from the character 
 of the Swiss Middle Tertiary flora, that 
 the mean temperature there (reduced to 
 sea level) must have been 70 to go C. 
 higher than now. This power of acclima- 
 tisation, resulting from inconstancy of con- 
 stitution and want of organisms, together 
 with the favourable influence of a uniformly 
 warm and damp climate, can scarcely 
 account even partly for this difference. 
 We must, therefore, take it for a fact that, 
 in former periods, as well as in the 
 tertiary period, the climates were warmer, 
 not only in Europe, Asia, and North 
 America, but also in the temperate zones 
 of the southern hemisphere. Setting aside 
 
IO 
 
 THE CLIMATES 
 
 local irregularities in the distribution of 
 heat, which existed then as now, the palae- 
 othermal phenomenon was a general occur- 
 rence which affected the whole of the 
 globe, but so that the difference between 
 the then and the present condition in- 
 creased in the direction of the poles. This 
 general greater equality of the climatic 
 conditions in past periods is a settled fact. 
 
 The Fossil Arctic Florae as evidence of 
 Warmer Climates. 
 
 The best proof for this is given by the 
 fossil flora of the arctic regions, the de- 
 scription and interpretation of which has 
 been one of the greatest achievements of 
 geological science, and would alone have 
 sufficed to immortalise the name, of Heer.* 
 
 * Chiefly by his two principal works : “ Die Urwelt der 
 Schweiz,” 1864 and 1879, and: “Flora Fossilis Arctica,” 
 1868, OswaldoHeer must be considered as the founder 
 
OF THE GEOLOGICAL PAST. I I 
 
 Gigantic Lepidodendra and Calamites, 
 trees related to our wolfs’ claws and mares’ 
 tails existed during the Carboniferous period, 
 not only in the temperate zones of the 
 
 of Palseoclimatology. To him we owe nearly all the infor- 
 mation which we possess about ancient climates. He pro- 
 ceeded with far greater caution in judging of the early dis- 
 tribution of heat over the earth from palaeontological data 
 than most of his successors, whose exaggerated conclusions 
 are often posed as his. 
 
 Between 1871 and 1883 six volumes more of his “Flora 
 Fossilis Arctica” were issued. 
 
 In a clear manner A. de Candolle, as early as 1855, 
 in his “Geographic Botanique Raisonnee,” and later in some 
 smaller papers, thoroughly discussed the relation between the 
 character of flora and climate, and the ideas of this great 
 botanist of Geneva exercised a strong influence over Heer’s 
 labours. 
 
 The first indications of a previous milder climate in higher 
 latitudes were found by A. Erman, in 1829, in Kamtchatka 
 at 63° N.L. (“Reise um die Erde,” Yol. 3. Berlin, 1848, 
 
 p. 149). 
 
 A small treatise of H. R. Goeppert (“ Ueber die Ter- 
 tiarflora der Polargegenden,” in : Bulletin de l’Academie Imp. 
 des Sciences de St. Petersbourg. T. 3. 1861, p. 448 — 461) 
 had remained almost unnoticed. 
 
THE CLIMATES 
 
 I 2 
 
 northern and southern hemispheres, but 
 also at Spitzbergen at 77^° N.L. Of the 
 26 species of Carboniferous plants which 
 were collected there, 1 7 existed during the 
 same period in Central Europe. Further, 
 a luxuriant Carboniferous vegetation existed 
 within the American north polar circle at 
 75 0 latitude, and in Siberia near the mouth 
 of the Lena at 71-J 0 N.L. 
 
 Fossil plants from the Permian and 
 Triassic period have not as yet been met 
 with in arctic regions, but two species of 
 Ichthyosaurus were found at Spitzbergen 
 at 78^° N.L. ; and marine mollusks, equal 
 to those which existed during that period 
 in temperate zones, were found chiefly 
 in North Siberia near the mouth of the 
 Olenek at 73 0 N.L., also at Spitzbergen. 
 
 In the Jurassic formation of Spitzbergen 
 32 species of mares’ tails, ferns, cycads, 
 and conifers were found at 78^-° N.L., 
 
OF THE GEOLOGICAL PAST. 
 
 13 
 
 10 of which were also met with in tem- 
 perate latitudes. 
 
 Of more importance, however, is our know- 
 ledge of the arctic Cretaceous formation. 
 Heer made us acquainted with the rich Cre- 
 taceous flora, which Nordenskiold col- 
 lected on the west coast of Greenland at 
 70 0 N.L., consisting of ferns, mares’ tails, 
 cycads, conifers, grasses, rushes, together 
 with one Dicotyledonous plant (Populus 
 primaeva) ; and, from somewhat more 
 recent beds, many Dicotyledonous trees, 
 such as magnoliae, different species of 
 fig-trees, sassafras and poplars, similar 
 to those found in the later Cretaceous 
 formation of Europe, where, however, 
 tropical and subtropical species are more 
 numerously represented. 
 
 Still richer is the Tertiary vegetation of 
 the arctic regions, which Heer inferred 
 to be Miocene, but to which some pa- 
 
14 
 
 THE CLIMATES 
 
 lseontologists, with Belt, Starkie Gardner 
 and de Saporta, now ascribe a somewhat 
 older, viz., Eocene, character. Heer des- 
 cribed Tertiary plants from Iceland, Green- 
 land, Grinnell Land, Spitzbergen, and North 
 Canada. Grinnell Land, in the great North 
 American archipelago, is the most northern 
 point of the globe, from which a fossil 
 flora is known. At 8 i 3 /4° N.L. Captain 
 Feilden, member of the Nares Arctic 
 Expedition, collected there 26 species of 
 plants, which were afterwards described by 
 Heer. Ten were found to be conifers, 
 the gigantic swamp-cypress (Taxodium 
 distichum), which grows at present in the 
 Southern United States, forming a large 
 portion of them. Together with these 
 grew poplars, birches, limes, elms, hazels 
 and a Viburnum, and in a lake enclosed 
 by the same forest, water-lilies, sedges 
 and rushes. A similar vegetation pre- 
 
OF THE GEOLOGICAL PAST. 
 
 15 
 
 vails at present in the northern tem- 
 perate zone with a mean yearly temperature 
 of 8° C; yet such a vegetation formerly 
 existed in a country where the mean tem- 
 perature is at present 20 0 — C ! 
 
 Heer described 179 species of Tertiary 
 plants from Spitzbergen, from 77^°to79° 
 N.L. , which belong to a flora almost similar to 
 that of Grinnell Land but pointing to a 
 slightly warmer climate. True cypresses 
 ( Libocedrus ) , willows, and not less than 
 seven species of poplars, together with 
 oaks, planes, magnolise, walnut-trees and 
 maples ; also cornelian, hawthorn and other 
 shrubs were found, and in a swamp, reeds, 
 orris roots, calamus and Potamogeton. This 
 indicates a flora similar to that which pre- 
 vails at present in Northern Germany with 
 a mean yearly temperature of 9 0 C. The 
 mean yearly temperature in that part of 
 Spitzbergen is now — 8° C. 
 
i6 
 
 THE CLIMATES 
 
 The rich Tertiary flora of Greenland 
 (especially at 70 0 N.L.) must have grown 
 in an even somewhat warmer climate. The 
 present mean temperature there is — 7 0 C. 
 There grew, side by side with many 
 other species of large forest trees, the 
 mammoth -tree (Sequoia), the Japanese 
 genus Gingko (Salisburea), chestnut-trees, 
 magnolise, evergreen oaks, laurels and a 
 vine. Heer points out that nowadays we 
 must travel 20 to 25 degrees of latitude 
 southwards, to the shores of the lake of 
 Geneva, to California and Japan, to meet 
 in Europe, North America and Asia, at a 
 mean yearly temperature of io^° C., a 
 similar vegetation. 
 
 Further west in the American arctic 
 archipelago, at Banks Land (74 0 N.L.) 
 and at Prince Patrick Island (76° N.L.) — 
 and further east at the Island of New 
 Siberia (at 75 0 and 75^-° N.L.), and at 
 
OF THE GEOLOGICAL PAST. 
 
 17 
 
 some other places within the Arctic circle, 
 numerous remains of a rich Tertiary forest 
 vegetation have been collected, from which 
 we may conclude that everywhere around 
 and near the north pole a much warmer 
 climate than the present must have pre- 
 vailed, and that the difference from the 
 actual condition was much more consider- 
 able there than we find it to have been 
 in temperate latitudes.* Whilst a rise in 
 temperature of 7 0 C. would have sufficed 
 to account for the Middle Tertiary vege- 
 
 *As far as Banks Land and Prince Patrick Island are 
 concerned some possibility remains that the trees found had 
 been floated there, though the discoverers themselves, Mac- 
 Clure and MacClintock, were of opinion that they had 
 grown on the place where they were found. According to 
 von Toll this must certainly have been the case in New 
 Siberia, for together with those trees brown coal and beds 
 consisting merely of the remains of leaves and fruit-cones 
 occur. (J. Schmalhausen, “ Tertiare Pflanzen der Insel 
 Neu-Sibirien,” Memoires de l’Acad. Imper. des Sc. de St. 
 Petersbourg. T. 37. 1890, No. 5.) 
 
 2 
 
1 8 
 
 THE CLIMATES 
 
 tation in Switzerland, it would have been 
 far from sufficient for the Arctic regions. 
 The difference from the present condition 
 was so great there, that we must assume 
 the temperature of Greenland, at 70 0 N.L., 
 and of Spitzbergen, at 78^° N.L., to have 
 been at least 17 0 C., and that of Grinnell 
 Land, at 8i-|° N.L., even 28° C. higher 
 than the actual temperature. In consider- 
 ing these figures we must keep in mind 
 that the climate of North America at pre- 
 sent is relatively cold compared to that of 
 Europe, and that, if the North American 
 archipelago were now favoured with warm 
 ocean currents like Spitzbergen and the 
 coast of Greenland, a rise of temperature 
 for Grinnell Land equal to that of the more 
 easterly Arctic regions would have sufficed. 
 
 We observe that, as a general rule, the 
 difference between the past and the actual 
 temperature of any climate increases, the 
 
OF THE GEOLOGICAL PAST. 
 
 19 
 
 nearer we approach the pole. Judging 
 by the character of Middle Tertiary plants 
 from Java, Borneo and Sumatra, described 
 by Goeppert, Heer, Geyler, Crie and 
 von Ettingshausen, and of Miocene sea 
 animals from Java, described by Martin, 
 the difference between the temperature 
 then prevailing near the equator with the 
 present can have been but trifling. Accord- 
 ing to certain observations made with 
 regard to the distribution of organisms, 
 especially land organisms in the Siluric, 
 the first half of the Coal period, and some 
 earlier periods, this difference may have 
 been a little greater inland during still 
 earlier periods. * 
 
 Very little is as yet known of early life 
 
 * According to later investigations by Heer (Compare: 
 C. Schroter, “ Oswald Heer, Lebensbild eines schweizeri- 
 schen Naturforschers.” Zurich, 1888, p. 335), the temperatures 
 in the Tertiary period (Lower Miocene and Oligocene of 
 Heer) were: 
 
20 
 
 THE CLIMATES 
 
 in high southern latitudes ; but judging by 
 the character of the fossil remains of plants 
 and animals found in temperate latitudes, 
 conditions in general similar to those 
 observed in the northern hemisphere must 
 have prevailed in Antarctic regions. * 
 
 Northern Italy 22 0 C. 
 
 Switzerland 20 '/a 0 
 
 Basin of the Lower Rhine . . . . 18 0 
 
 Dantzig 17 0 
 
 Greenland at 70° N.L 12 0 
 
 Spitzbergen at 78° N.L 9 0 
 
 Grinnell Land at 82° N.L 8° 
 
 *The Tertiary flora of New South Wales and of New 
 
 Zealand had, for instance, the same climatic character as that 
 of Europe, and was even closely related to it. (Von 
 Ettingshausen, Denkschr. K. Akad. Wien, Math. Nat. 
 Cl. 1886. Vol. 53, p. 79, and Ibid. 1887, p. 140. — From 
 the Tertiary formation in Kerguelen Island L. (49 0 S.L.) Crie 
 (Palseont. Abhandl. von Dames und Kayser, N. F. Vol. 1. 
 Heft 2. 1889) described large stems of trees ( Cupressoxylon ), 
 and from Punta Arenas (53 ^'a 0 S.L.), at the Strait of 
 Magellan, H. Engelhardt (Abhandl. der Senckenberg.. 
 naturf. Gesellsch. Frankfurt a. M. 1891) Vol. 16, pp. 629 — 
 672 a Tertiary florula of tropical character. 
 
OF THE GEOLOGICAL PAST. 2 1 
 
 An attempt to explain that even ever- 
 greens and shrubs could thrive in 
 Arctic regions, notwithstanding the long 
 polar night, is made by Heer from a 
 comparison with present facts ; namely, 
 that even now some trees and shrubs 
 survive the long polar night near the 70 th 
 degree of northern latitude ; that many 
 tropical plants live during the long win- 
 ter in St. Petersburg in hot-houses, while 
 the amount of light received is but very 
 small; and that evergreen alpine shrubs, 
 mountain-pine (Pinus pumilis), rhododen- 
 drons and heather are likewise for many 
 months cut off from all light by a covering 
 of snow. To these arguments we may 
 add that the polar night is considerably 
 shortened by a very strong twilight, * 
 that the lighting power of solar rays at 
 
 * Compare: J. Hann, “ Handbuch der Klimatologie.” 
 Stuttgart, 1883, p. 749. 
 
22 
 
 THE CLIMATES 
 
 a low angle of incidence decreases con- 
 siderably less than the heating power* 
 and that lastly, and perhaps chiefly, though 
 the arctic summer be of short duration* 
 the quantity of solar light received 
 is comparatively large, and equal to half 
 the quantity received in the same length 
 of time near the equator.* The arctic 
 plants are thus enabled to store a large 
 supply of fecula, by which they can bring 
 buds and leaves to rapid development, as 
 soon as the temperature rises in the spring. f 
 Near the pole the solar radiation in 
 midsummer is daily greater than it ever 
 is at the equator. The greater part of this 
 radiant heat is now employed in melting 
 
 *R. Spitaler, “Die Lichtvertheilung auf der Erdober- 
 flache,” in: Eder’s Jahrbuch fur Photographie, 1888. 
 
 f Compare the picture of the relatively luxuriant present 
 vegetation in the interior of Grinnell Land, drawn by A. 
 W. Greely in : “ Three Years of Arctic Service.” London* 
 1886. 
 
OF THE GEOLOGICAL PAST. 23 
 
 the ice and the large quantities of snow 
 which have been formed and have accu- 
 mulated during the long frosty seasons. 
 How entirely different would vegetation 
 flourish in polar regions if during the 
 whole year warm sea currents prevented 
 the formation of the huge ice masses, and 
 mild winds continued blowing during the 
 long winter night ! * 
 
 Climatic Zones of former Periods. 
 
 One of 'the most important results of 
 Heer’s investigations is that the Tertiary 
 arctic flora forms a circuit around the pole, 
 which fact caused that great phyto-palae- 
 ontologist to reject emphatically any dis- 
 placement of the earth’s axis of rotation 
 and geographical displacement of the poles, 
 suggested as an explanation of the Tertiary 
 distribution of heat. 
 
 Compare: Hann, “Handbuch der Klimatologie,” p. 744. 
 
24 
 
 THE CLIMATES 
 
 He also proved the existence of climatic 
 zones concentric around the present pole 
 during the Tertiary and the last half of 
 the Cretaceous periods. He was of opinion 
 — an opinion that is now still too widely 
 spread — that in the former periods over 
 the whole surface of the earth a uniform 
 tropical climate prevailed, though with a 
 range of the mean yearly temperature 
 from 20° to 2 5° C. The groundlessness 
 of the assumption of a perfectly uniform 
 distribution of heat during the Pretertiary 
 ages has since been recognised. Besides 
 irregular climatic anomalies in earlier 
 periods, a distinct northern and southern 
 facies of the fauna has been observed to 
 have existed during the Cretaceous period 
 (F. Romer 1852), which must be ascribed 
 to a similar, though less pronounced dis- 
 tribution of heat in zones between the 
 equator and the poles as now exists. 
 
OF THE GEOLOGICAL PAST. 25 
 
 (Xrc y 0 1 / 5 ' rt/,v Of/j ? 
 
 Neumayr,* further, amply proved the 
 existence of heat belts during the Jurassic 
 period. By more special investigation of 
 the distribution of marine animals he suc- 
 ceeded in recognising a tropical zone, 
 which reached southward as far as the 
 20 th degree of southern latitude and included 
 northward the southern part of Europe; 
 a temperate (as regards climate: subtropi- 
 cal) zone, which reached farthest north in 
 a large sea arm to the west of Scandi- 
 navia, corresponding to the present Atlantic 
 Ocean; and a boreal (as regards climate: 
 warm temperate) zone which included the 
 highest northern latitudes. 
 
 During the entire Jurassic period which, 
 judging by the changes observed in its 
 
 * M. Neumayr, “ Ueber klimatische Zonen wahrend der 
 Jura- und Kreide-Zeit.” Denkschriften der K. Akademie 
 der Wissensch. zu Wien. Math. Nat. Cl. 1883. Vol. 4 7, 
 and: Die geographische Verbreitung der Juraformation. Ibid. 
 1885, Vol. 50, p. 57. 
 
2 6 
 
 THE CLIMATES 
 
 organisms, must have lasted much longer 
 than the Tertiary period, and also during 
 the Cretaceous period, these limits remain 
 on the whole unaltered, notwithstanding 
 the considerable changes which took place 
 in the distribution of land and sea. This 
 proves how little influence distribution 
 exercises on the general diffusion of heat 
 over the surface of the earth, at least as 
 far as the temperature of the sea water 
 (and sea climate) is concerned. 
 
 Probably during earlier periods similar 
 zones, with presumably still less mutual 
 differences in temperature, existed — and 
 it is quite possible that some day we may 
 succeed, in other parts of the world, in 
 proving their existence, in the same way 
 as that of the zones during the Jurassic 
 and the Cretaceous periods in Europe. 
 As has been rightly said by Neumayr the 
 geological circumstances are too unfavour- 
 
OF THE GEOLOGICAL PAST. 
 
 27 
 
 able in Europe for arriving at an estimation 
 of the climates in earlier periods. It would 
 really seem as if the Coal flora and the 
 fauna of the Carboniferous limestone in 
 high northern latitudes differed slightly 
 from those of Europe, by the scarcity or 
 absence of Sigillarice and the scarcity o i 
 Fusulince. On account of the almost total 
 stability observed during the greater part 
 of the Mesozoic period we may assume as 
 probable that the distribution of heat since 
 the beginning of the Palaeozoic period also 
 has undergone but slight changes. So 
 much is now manifest, that a lower 
 temperature in Europe and North America 
 during the Coal period than the present 
 temperature in the tropical zone would 
 best explain the observed phenomena; but 
 it would be too much to infer that a New 
 Zealand forest is the best recent illustration 
 of the old Coal vegetation. Not the three 
 
28 
 
 THE CLIMATES 
 
 ferns and conifers preponderating there, 
 but trees, having succulent stems, whose 
 near relations, the present herbaceous Equi- 
 setacecc and Lycopodincc, are mostly inhabi- 
 tants of the tropics, we must consider as 
 the chief types of the Carboniferous forma- 
 tion. We might somewhat more rightly 
 compare the Coal flora to the swamp 
 forests of gigantic Equisetacea near Caracas 
 in Venezuela. The Carboniferous ferns 
 differ considerably from the present types, 
 but half the number of their species were 
 related to the recent tropical family, the 
 Marattiacece. Every closer comparison of 
 that early flora with plants now existing 
 must necessarily fail, through their being 
 too distantly related, but we may safely 
 conclude from the general character of the 
 Carboniferous flora that those old crypto- 
 gamiae with their gigantic but succulent 
 stems, with little wood fibre, sparingly 
 
OF THE GEOLOGICAL PAST. 
 
 29 
 
 branched and having short, stiff, grass-like 
 leaves, may have been fit to resist a strong 
 wind but certainly not even the slightest 
 frost. The fact that these existed in tem- 
 perate latitudes and even in the polar region 
 seems to prove conclusively that there, 
 during the Palaeozoic period, not only a 
 more moist but also a considerably warmer 
 and more uniform climate than the present 
 must have prevailed. A proof, almost 
 equal to an experiment, for the correctness 
 of the assumption that a mild climate has 
 been actually a vital necessity for the Coal 
 flora, is given by what took place at the 
 incidence of glacial phenomena in South 
 Africa and Australia during the last part 
 of the Carboniferous period. With the 
 approaching cold we see the real Palseozoic 
 Coal flora, which at that time was every- 
 where else at its highest state of development, 
 immediately and entirely disappear there 
 
30 
 
 THE CLIMATES 
 
 and give place to a flora which appeared 
 in the Triassic period* in Europe much 
 later on. 
 
 Not only the distribution of plants, gener- 
 ally more directly dependent upon the 
 climate for existence than is the case with 
 animals, but also the distribution of reef- 
 building corals has long been rightly con- 
 sidered an important phenomenon by which 
 to judge of early climates; corals being 
 characteristic tropical organisms. Reef- 
 
 *W. Waagen, “The Carboniferous Glacial Period. Rec. 
 Geol. Surv. Ind.” Vol. 21. 1888, p. 89. — Compare also: 
 O. Feistmantel. “Ueber die pflanzen- und kohlenfuhrenden 
 Schichten in Indien, Afrika und Australien und darin vor- 
 kommende glaziale Erscheinungen.” Sitz. Ber. Bohm. Ge- 
 sellsch. d. Wiss. 1887, p. 3, and: “Ueber die geologischen 
 und palaeontologischen Verhaltnisse der im Gondwana-System 
 vorkommenden Arten.” Sitz. Ber. Bohm. Ges. d. Wiss. 
 1889, p. 584 — 054, further : “ On the Coal and Plant-bearing 
 
 Beds of Palasozoic and Mesozoic Ages in Eastern Australia 
 and Tasmania ; with special reference to the fossil Flora. 
 Mem. Geol. Surv. New South Wales.” 1890. 
 
OF THE GEOLOGICAL PAST. 
 
 31 
 
 building coral zoophytes live at present 
 only in warm seas, the surface temperature 
 of which never falls below 20° C., and 
 where the annual range of temperature is 
 not greater than 7 0 C. This accounts for 
 the fact that they are found as far as the 
 Bermuda Islands at 32 0 N.L. where the 
 influence of the warm Gulf Stream is still 
 very noticeable, and not near the Galapa- 
 gos Islands under the equator, the coasts 
 of which are washed by the cool Peru 
 current. Enormous old coral reefs are 
 known from the Silurian and Carboniferous 
 periods of Europe, North America and 
 many other regions in temperate and high 
 latitudes. Within the Arctic circle Silurian 
 reef-building corals have been found at 
 North Devon and Beechy Island. But 
 the type of these Palaeozoic corals differs 
 very essentially from those now existing; 
 for all this, it might be possible that they 
 
32 
 
 THE CLIMATES 
 
 had accommodated themselves to condi- 
 tions different from those under which the 
 present reef-builders exist, though on phy- 
 siological grounds we can scarcely think 
 that this difference can have been very 
 considerable.* 
 
 Remains of old coral reefs from the 
 Triassic period exist in Central Europe. 
 
 The temperature of early seas may be 
 concluded with greater certainty from the 
 distribution of coral reefs in the Jurassic 
 and Tertiary formation, as these belong to 
 the present type. Those of the Jurassic 
 period reach as far north as Northern 
 Germany and Central England. In the 
 Cretaceous formation and in the first half 
 of the Tertiary period their limit was but 
 
 s. * Because the secretion of lime by the organisms is in a 
 high degree dependent on the temperature. See: Murray 
 and R. Irvine, “On Coral Reefs and other Carbonate of 
 Lime Formations in Modern Seas.” Proceed. Royal Society 
 of Edinburgh. Vol. 17. 1890, pp. 80 — 82 and p. 90. 
 
OF THE GEOLOGICAL PAST. 33 
 
 a little more southerly, while on the contrary 
 in other parts of the world their limit was 
 farther from the equator. General changes 
 (having affected the whole of the globe) 
 did not occur — as far as we know — during 
 the Cretaceous period. 
 
 Local Anomalies in Climatic Conditions. 
 
 The climate of North America was in 
 the Jurassic period already somewhat cooler 
 than that of Europe, a difference which 
 became still more apparent in the Creta- 
 ceous period, and while an almost tropical 
 climate prevailed in Central Europe during 
 the Eocene period, the Eocene flora of 
 the North American continent bore a sub- 
 tropical or moderately warm character. 
 During the Tertiary period, at least, the 
 climate of Europe compared to that of 
 other regions must have been abnormally 
 warm and that of the interior of North 
 
 3 
 
34 
 
 THE CLIMATES 
 
 America abnormally jcooI — a difference 
 entirely analogous to that observed nowa- 
 days, which in all probability is attri- 
 butable to the same causes. ^ Already 
 in very early times North America formed 
 an extensive continent, which developed to 
 its present condition during the transition 
 of the Cretaceous to the Tertiary period 
 when the last extensive bays receded. It 
 is therefore obvious that, under the same 
 general conditions of heat on the earth as 
 prevail at present, its climate must have 
 been quite different from — very much cooler 
 than that of Europe, — for this consisted 
 for long ages of islands and peninsulas 
 with extensive inland seas and large bays 
 into which warm water was driven from 
 southern regions by the prevailing winds, 
 rendering the climate a very mild one 
 even as far as the polar sea. It seems 
 probable, moreover, that during long ages, 
 
OF THE GEOLOGICAL PAST. 
 
 35 
 
 and as late as the Tertiary age, extensive 
 connections existed through Russia and 
 Siberia between the arctic seas and the warm 
 seas of Europe and Southern Asia. Hence, 
 even for the arctic regions of North East 
 Asia and North America more favourable 
 conditions must have prevailed than at 
 present. This explains the fact that in Central 
 Asia at the foot of the Altai mountains a 
 true Tertiary forest vegetation could exist.* 
 Local anomalies in the general distribu- 
 tion of heat obtained in every former period 
 the same as they do now, and so many 
 geologists have found evidences of ice 
 action in different parts of the globe and 
 so nearly in the complete series of forma- 
 tions, that these cannot well be passed 
 over unobserved. *j* Those earlier ice forma- 
 
 * Schmalhausen and Maximo wicz in ; Palseontographica. 
 Beitrage zur Naturgeschichte der Vorzeit, herausgeg. von 
 K. A. von Zittel. Vol 33. Stuttgart 1885, pp. 142 — 216. 
 
 tA summary of those cited pre-Diluvial glaciations may 
 
36 
 
 THE CLIMATES 
 
 tions, however, differ in so far from the 
 Pleistocene glaciation, that they do not 
 occur as a general phenomenon, but scat- 
 tered over various parts of the globe, 
 apparently independent of any special lati- 
 tude, while at the same time organisms 
 indicative of a warm climate existed in 
 other regions under very different latitudes. 
 From the last half of the Carboniferous 
 period (according to some, the first Per- 
 mian period) extensive and thick beds 
 exist in South Africa, India and Australia, 
 which do not contain Sigillarise, Lepido- 
 dendra, Calamites, or other typical coal 
 plants, but in which is found a flora of 
 ferns, chiefly belonging to the genus 
 Glossopteris , with its allies Gangamopteris 
 and Noggeratk iopsis, together with Equise- 
 tacese, some Conifers and Cycads. This 
 
 be found in: Croll, “Climate and Time.” London, 1875. 
 Chapter XVIII. 
 
OF THE GEOLOGICAL PAST. 
 
 37 
 
 vegetation, as above mentioned, did not at 
 all belong to the Carboniferous flora, but is 
 nearly related to that which was to appear 
 later on in Europe during the Triassic 
 period. This new flora is found in South 
 Africa, India and Australia together with 
 deposits which to all appearance have been 
 formed by ice action.* During the same 
 
 ♦See: W. Waagen, “The Carboniferous Glacial Period.” 
 l.c. ; O. Feistmantel, “Ueber die pflanzen-und kohlenfuhrenden 
 Schichten in Indien, Afrika und Australien, l.c., etc.” 
 
 According to later communications traces of a Carbonifer- 
 ous glaciation were supposed to have been found in Brazil. 
 “ (The Carboniferous Glacial Period.” Further Note by Dr. 
 W. Waagen on a letter from Mr. A. Derby. “ Records 
 of the Geol. Survey of India.” Vol. 22. 1889, p. 69). The 
 presence of large blocks of granite and gneiss in fine clay 
 can, however, be explained by other causes than ice-trans- 
 port. As long as no clearly polished rocks and scratched 
 stones have been found it will be prudent to consider the 
 occurrence of glaciation during the Carboniferous formation 
 of Brazil (though this contains very considerable beds of 
 Coal) as at least very doubtful. For pseudo-glacial pheno- 
 mena see: A. Penck in: “Das Ausland” 1884, p. 641 sqq. 
 
38 
 
 THE CLIMATES 
 
 time the luxuriant Carboniferous vegetation 
 existed in Europe, Asia and North America, 
 and even close to the north pole, and the 
 Carboniferous marine fauna remains unal- 
 tered. This remarkable phenomenon can, 
 therefore, not be explained by one general 
 cause, the influence of which was felt over 
 the whole earth. Here again a displace- 
 ment of the earth’s axis and the poles has 
 been assumed.* On closely investigating 
 the effect of such a displacement it be- 
 comes apparent that, though for one of the 
 poles a geographical position might be 
 suggested by which the simultaneous exist- 
 ence of ice and a nearly tropical vegeta- 
 tion on different parts of the earth, where 
 quite opposite circumstances obtain at 
 present, could be made somewhat less 
 obscure, yet this suggestion would neces- 
 
 *H. B. Medlicott and W. T. Blanford, “Manual of 
 the Geology of India.” Part. i. Calcutta, 1879, p. xxxvii. 
 
OF THE GEOLOGICAL PAST. 
 
 39 
 
 sarily put the opposite pole in the middle 
 of a typical Coal flora. Further, the dis- 
 tance of the ice-covered regions from the 
 hypothetical south pole is so great that 
 the temperature of the whole earth ought 
 to have been much lower than it is at 
 present, which certainly was not the case. 
 It seems quite impossible to explain these 
 phenomena by general causes, and we are 
 obliged to ascribe even those extensive 
 climatic anomalies to local influences. Now, 
 it has been proved that the existence of 
 ice in the form of glaciers depends much 
 more upon the dampness of the climate 
 than upon its low temperature. In the 
 moist regions of New Zealand, with a mean 
 yearly temperature of io° C. — a climate 
 equally as warm as that of Vienna and 
 warmer than that of Geneva — glaciers de- 
 scend as low as 213 metres above the sea 
 level, in between dense forests with fern,. 
 
40 
 
 THE CLIMATES 
 
 trees, palms and fuchsias, while in the dry 
 mountain ranges of Eastern Siberia, with 
 a mean yearly temperature varying from 
 
 — 1 5° C. to — 1 8° C. , neither permanent snow 
 nor glaciers are found. On the southern 
 slopes of the Himalayas (with a mean 
 yearly temperature of +0.5 0 C. at snow 
 line) permanent snow and glaciers descend 
 more than 700 metres lower than on the 
 northern slopes (with a temperature of 
 
 — 2.8° C. at snow line). The yearly temper- 
 ature near the limit of permanent snow 
 is in the tropics +1.5 0 C. and in the 
 Swiss Alps about — 3 0 C. It has also been 
 observed that the periods during which 
 the glaciers in the Alps increase may coin- 
 cide with warm years, and that on the 
 contrary periods during which glaciers 
 decrease may correspond with cold years ; 
 but an increase of glaciers takes place 
 when during some years moist westerly 
 
OF THE GEOLOGICAL PAST, 
 
 41 
 
 winds have prevailed, and a decrease when 
 easterly and northerly winds have been 
 prevalent.* These and other phenomena 
 justify the conclusion that the main factor 
 in the formation of glaciers is the amount 
 of snow fallen in the fern region. If in 
 early periods, and as late as in the Terti- 
 ary period, heat was distributed far more 
 uniformly over the earth, without any 
 decrease of temperature being observable 
 in the equatorial regions, tellural causes 
 being, further, totally inefficient in them- 
 selves to affect any considerable change 
 in the general temperature of the globe, 
 then it is obvious that the total amount 
 of heat received by the earth must have 
 been greater. The atmosphere must then 
 have contained a greater amount of vapour, 
 
 * C. Lang, “ Der sakulare Verlauf der Witterung als Ursache 
 der Gletscherchwankungen in den Alpen.” Zeitschr. der 
 Oesterr. Gesellsch. fur Meteorologie. 1885. Vol. 20, p, 443. 
 
42 
 
 THE CLIMATES 
 
 and in places where a considerable upheaval 
 of the surface of the earth existed snow 
 and ice could accumulate to form glaciers 
 more easily than at present. In regions 
 which were as warm then as the tropics 
 now are the lower limit of permanent snow 
 may have been below the present (5000 
 metres) and the upper limit, on the con- 
 trary, above it. With an abundant supply 
 of snow it may then have been possible 
 that in mountainous regions glaciers were 
 formed, which under very favourable cir- 
 cumstances descended even as low as the 
 sea level. Just as during the Carbonife- 
 rous age an extensive lowland, cut up by 
 the sea into a large marshy archipelago, 
 accounts for the formation of coal over 
 nearly the whole of the northern hemis- 
 phere, to such an extent that comparison 
 can only be made with the extensive depo- 
 sits of Jurassic coal, extending from Western 
 
OF THE GEOLOGICAL PAST. 
 
 43 
 
 Asia to Australia, it seems that a large 
 mountainous continent (“ Gondwana Land ” 
 of Suess), at and south of the equator, has 
 caused extensive accumulations of ice in 
 suitable places. A great uniformity of 
 orographic conditions over extensive con- 
 tinental parts of the earth’s crust seems to 
 have been characteristic of the Coal period. 
 It is thus possible, and even probable, that 
 by a gradual upheaval of such a continent, 
 the changed conditions of existence caused 
 the development of a new flora, which 
 only much later, in the beginning of the 
 Mesozoic period, should find in Europe, 
 in the higher upheaval of the ground, con- 
 ditions it was better fitted for than was 
 the older Palaeozoic flora which in conse- 
 quence would suffer extermination. Traces 
 of glaciation are believed to have been 
 actually found in the Permian formation 
 of Europe. From those high centres of 
 
44 
 
 THE CLIMATES 
 
 acclimatisation the new flora, accommodat 
 ing itself to a higher temperature, could 
 then have gradually spread over the low- 
 lands. 
 
 Many particulars in the distribution oT 
 organisms of other periods also point to 
 differences of temperature in regions not 
 far distant from one another, and as no 
 difference in latitude can account for these 
 anomalies, they must be attributed to local 
 influences similar to those caused in our 
 days by the distribution of land and sea, 
 the elevation of the land, the direction of 
 winds, and the neighbourhood of cold or 
 warm ocean currents. As far, however, 
 as we were able to detect general regu- 
 larity in the distribution of heat, with 
 certainty, since the Jurassic period, 
 the distribution then was similar to that 
 now prevailing, and formed concentric 
 belts round the poles, differing only from 
 
OF THE GEOLOGICAL PAST. 
 
 45 
 
 the present by their mutual limits and 
 differences of temperature. 
 
 The Hypothesis of a Tertiary Displacement 
 of the Pole unnecessary, and insufficient 
 to explain the Phenomena. 
 
 It was already known to Heer that the 
 Tertiary arctic flora formed a chain round 
 the pole from which it could nowhere 
 escape. Neither would the supposition of 
 a displacement of the pole account here 
 for the palseothermal phenomenon;* but, 
 
 * An acccount of the earlier attempts to explain the warm 
 arctic climates by the secular changes in the position of the 
 earth’s axis has been given by F. von Czerny (“Die Ver- 
 anderlichkeit des Klimas und ihre Ursachen.” Wien. Pest. 
 Leipzig 1881, p. 86 sqq.). S. Haughton. as early as 1878 
 (“Physical Geography, Nature.” Vol. 18, p. 266 sqq.) proved 
 on geological grounds the impossibility of this explanation. — 
 With regard to the mechanical side of this question it is 
 now certain that any important deviations of the poles 
 can only be caused by geological processes if the rigidity of 
 the earth is not very considerable. See: G. V. Schiaparelli. 
 
46 
 
 THE CLIMATES 
 
 as this could be explained by no other 
 hypothesis, the almost-forgotten displace- 
 ment of the pole was again in recent times 
 seized upon, just as a drowning man grasps 
 at a straw. Encouraged by new astro- 
 nomical data, by which very small displace- 
 ments of the earth’s axis were proved, 
 Neumayr and Nathorst considered it 
 warrantable to attempt an explanation of 
 the polar regions, at least partly, by si- 
 milar, but larger, displacements.* It is, 
 however, not only very questionable whether 
 one is justified in assuming a lasting 
 displacement of ten and more degrees, 
 
 “ De la rotation de la Terre sous l’influence des actions geo- 
 logiques. Memoir e presente k l’Observatoire de Poulkova a 
 l’occasion de sa fete semiseculaire. St. Petersbourg 1889. 
 
 *M. Neumayr, Erdgeschichte. Vol. II, p. 512 sqq. and: 
 “The Climates of Past Ages. Nature.” Vol. 42. 1890, p. 171. 
 
 A. G. Nathorst, “ Zur fossilen Flora Japan’s. Palseont. 
 Abhandlungen herrausgegeben von Dames und Kayser.” 
 Vol. 4. 1888. 
 
OF THE GEOLOGICAL PAST. 
 
 47 
 
 merely from the fact that a fourteen months 
 periodic displacement of parts of seconds 
 wanderings with an amplitude of 15 M. 
 have been proved, but the data of palae- 
 ontology are not in harmony with such a 
 supposition. Neumayr was led to that 
 supposition chiefly by the existence of a 
 luxuriant Tertiary flora in Grinnell Land, 
 and by the fact that the Tertiary flora of 
 Alaska, at 6o° N.L., scarcely bears the 
 character of a southern flora any more 
 than that of Spitzbergen at 78° N.L. does, 
 and also that outside the polar circle the 
 distribution of organisms seems to point to 
 such a displacement. In fact, the Tertiary 
 flora of the interior of North America 
 bears the character of a subtropical or 
 even of a moderately warm climate, and 
 the Later Tertiary plants of Mogi in the 
 Japanese island Kiu Shiu, described by 
 Nathorst, point to a somewhat cooler 
 
48 
 
 THE CLIMATES 
 
 climate than the present. Neither could 
 Darwin or Philippi conclude from the 
 Tertiary marine animals found off the coast 
 of Chili, at 35 0 S.L. a temperature of the 
 sea water higher than the present compa- 
 ratively cold one.* 
 
 It will no doubt be found striking that 
 all these regions have now also a compa- 
 ratively cool climate. Since in the Tertiary 
 period the present continents (Europe ex- 
 cepted) had, at least in those parts of 
 North America, South America and East- 
 ern Asia, almost the same outlines as they 
 have now, it will be more natural to seek 
 the cause of these Tertiary anomalies in 
 
 *Ch. Darwin, Geological observations on the volcanic 
 Islands and Parts of South America, visited during the 
 voyage of H. M. S. “Beagle”, 3 rd Edition, London 1891, 
 p. 413. The “Observations on South America” appeared 
 for the first time in 1846. 
 
 R. A. Philippi, Die tertiaren und quartaren Versteiner- 
 ungen Chile’s. Leipzig, 1887. 
 
OF THE GEOLOGICAL PAST. 
 
 49 
 
 the general distribution of heat in influences 
 which account for those anomalies nowa- 
 days, than in hypothetical considerable 
 displacements of the earths axis. The 
 climate of an extensive continent like that 
 of Tertiary North America, not directly 
 influenced by warm ocean currents, must 
 have been less fit for a subtropical vege- 
 tation than the insular climate of Europe, 
 influenced as this was by warm currents, 
 and the difference between the climates of 
 these two parts of the world must have 
 been still more considerable than it is now. 
 It may have been that during the Pliocene 
 period, when the general distribution of 
 heat differed but litde from that of the 
 present time, the jlimate of Japan was 
 somewhat cooler than it is now, if by the 
 connection of the islands with the conti- 
 nent the climate was less insular and in 
 that way the influence of the cold Oja-Shio 
 
 4 
 
50 
 
 THE CLIMATES 
 
 current on the east coast was stronger, 
 while the influence of that branch of the 
 warm Kuro-Shio current, which nowadays 
 washes the west coast, would not exist.* 
 The west coast of South America 
 owes its present relatively cool climate 
 to the influence of the cold Peru current. 
 As will be more fully explained in the 
 second Part, the chief conditions of a greater 
 intensity of all the ocean currents, and 
 therefore of this current likewise, existed 
 in the Tertiary period. Its greater power 
 could thus compensate its relatively warmer 
 origin, so that the sea water by which the 
 Tertiary coast of Chili was washed was 
 not warmer than it at present (14 1 j^° C.).f 
 
 * On these influences acting on the present climate of 
 Japan see : A. Woeikof, Bemerkungen iiber die Temperatur 
 der ostasiatischen Inselreihe Sachalin, Jesso und Nippon. 
 Zeitschr. fur osterr. Meteorologie. Vol. 20, 1885, pp. 1 — 3, 
 and: Die Klimate der Erde. Jena, 1887. Vol. II, p. 365 sqq. 
 
 f According to E. Suess (Das Antlitz der Erde. Vol. 2. 
 
OF THE GEOLOGICAL PAST. 
 
 51 
 
 The cooler Tertiary climate of Alaska 
 may also be explained by the same cause 
 which renders it relatively cool at present : 
 namely, predominating land winds, f 
 
 Moreover, a displacement of the pole 
 would not suffice to render the Tertiary 
 arctic phenomena any more intelligible to 
 us. Neumayr supposes a displacement of 
 the pole from io° to 20 0 along the Ferro 
 meridian in the direction of Eastern Asia. 
 A displacement of io° in that direction 
 
 Wien. Prag. Leipzig, 1888, p. 661) it is, moreover, still 
 questionable whether the expression “ Tertiary ” applied to 
 the respective marine deposits in Chili is really equivalent 
 to the European term “ Tertiary ”. H. Engelhardt (Ab- 
 hand. der Naturf. Gesellsch. Isis. Dresden 1890, p. 3, and: 
 Abhandl. der Senckenberg. naturf. Gesellsch. Frankfurt a. M. 
 1891) described Chilenian Tertiary plants — even from Punta 
 Arenas at the Magellan Strait — which point to a damp tro- 
 pical climate. 
 
 *W. H. Dall, Alaska Meteorology. Coast Pilot of 
 Alaska. App. I. Washington, 1879. — Compare further: 
 Hann’s Isobarenkarte, No. 6 of the Abtheilung Meterologie 
 in: Berghaus’ Physikal. Atlas Gotha, 1887. 
 
52 
 
 THE CLIMATES 
 
 would certainly place Grinnell Land, Green 
 land and Spitzbergen in more favourable 
 conditions, but the Tertiary forests of New 
 Siberia would then come even in closer 
 proximity to the pole than Grinnell Land 
 is at present. A larger displacement would 
 lead to still more impossible results, and 
 a much smaller one would produce no 
 noticeable effect. The opinion expressed 
 by Nathorst, that the Tertiary north pole 
 has been situated on the meridian 120 0 
 eastern longitude of Greenwich, 20 0 more 
 towards Asia, is in opposition to the 
 discoveries lately made in New Siberia,* 
 and also to those of the Tertiary florula of 
 Punta Arenas, f which then would have 
 existed at about 73-J- 0 S.L. Neumayr 
 
 * Compare : E. von Toll in: J. Schmalhausen, Ter- 
 tiare Pflanzen der Insel Neu-Sibirien, l.c., pp. 7 — 9. 
 
 j-H. Engelhardt, Abhandl. der Senckenberg. Naturf. 
 Gesellschaft, Frankfurt a. M. 1891. 
 
OF THE GEOLOGICAL PAST. 
 
 53 
 
 himself must acknowledge that for any 
 imaginary position of the pole those places 
 where luxuriant Tertiary forests existed 
 remain in much closer proximity to the 
 pole than the present most northerly limit, 
 where stunted trees are to be found, and 
 that in any case the fossil flora of Europe 
 deviates much more towards a warmer 
 climate, than that of Japan towards a cooler 
 climate than the present. 
 
 Should, therefore, astronomers and phy- 
 sicists raise no objections to a considerable 
 displacement of the poles, the facts observed 
 with regard to the arctic vegetation of the 
 Tertiary period still offer sufficient argu- 
 ments against such a supposition. The dis- 
 placements of the earth’s axis and of the 
 poles, if any have taken place at all in 
 early periods, can have been but trifling, 
 for the climatic distribution of organisms 
 during those periods which we have been 
 
54 
 
 THE CLIMATES 
 
 able to examine in this respect, corresponds 
 with zones which concentrically surround 
 the present poles. * 
 
 We may thus conclude that, from the 
 age of the first Palaeozoic organisms up to 
 the end of the Tertiary period, in high 
 latitudes, either in the northern or southern 
 hemisphere (temporary local anomalies not 
 taken into account), warm climates pre- 
 vailed, and that the deviation from the 
 present conditions increased towards the 
 poles. As far as we are able to ascertain, 
 
 * Neumayr himself considers it “ a fact of great impor- 
 tance ” that, according to his investigations, the limits of the 
 homoeozic belts in the Jurassic period ran almost parallel to 
 the present equator of the earth, which proves that “the 
 equator and the poles cannot have undergone any consider- 
 able displacement since the Jurassic period ” . “ the results, 
 
 here obtained oppose conclusively the assumption that since 
 the Jurassic period displacements in that direction large 
 enough to account for considerable displacements of the 
 zoogeographic zones have taken place.” (Ueber klimatische 
 Zonen wahrend der Jura- und Kreidezeit. 1 . c. p. 307). 
 
OF THE GEOLOGICAL PAST. 
 
 55 
 
 the regions near the equator have under- 
 gone but slight changes, and have (tem- 
 porary local anomalies not taken into 
 account) never been colder but perhaps 
 slightly warmer than at present. 
 
 Gradual Tertiary Cooling of the Climates, and 
 Pleistocene Glacial Epochs. 
 
 During the relatively short Tertiary 
 period a general cooling gradually took 
 place. Evidently under more favourable con- 
 ditions, resulting from the distribution of 
 land and sea, this cooling commenced in 
 Europe somewhat later, but then proceeded 
 more rapidly than in North America. As, 
 however, in the latter continent the above- 
 mentioned local conditions underwent but 
 relatively little change from the commencement 
 of the Tertiary period, we must conclude 
 that the general cause from which this 
 cooling originated would act there least 
 
5 & 
 
 THE CLIMATES 
 
 disturbed, and that, precisely as has been 
 observed in North America, this general 
 cooling really commenced at the end of 
 the Cretaceous period and lasted up to the 
 end of the Tertiary period. The gradual 
 decrease of heat going on during this 
 whole period and the continual approach 
 to the present climatic conditions find their 
 unmistakeable expression in the change of 
 the organic world, notably in its distribu- 
 tion. These will be generally acknowledged 
 facts. 
 
 By the assumption that not earlier than 
 the Tertiary period a considerable cooling 
 of the previously almost homogeneous 
 climate of the earth set in over the greater 
 half of the earth’s surface, an otherwise 
 totally obscure chief feature in the palae- 
 ontological history of the vertebrate becomes 
 clear. For it is only at the end of the 
 Mesozoic period that we see the reptiles, 
 
OF THE GEOLOGICAL PAST. 
 
 57 
 
 the dominators of that long age, possessing 
 scarcely any heat of their own, and being 
 but poorly protected against changes of 
 temperature by their scales and shells, 
 recede rapidly before the classes of mam- 
 mals and birds. Though these had already 
 appeared very early in single, small and 
 low forms, they first attained, and that 
 rapidly, a high flourishing state in the 
 Tertiary period. For these classes possess 
 in their warm blood and their bodily co- 
 vering as well as in the development of 
 their eggs — which, unlike that of those 
 coldblooded vertebrates, requiring for this 
 development, as for their whole vital acti- 
 vity, external heat so much, is not di- 
 rectly dependent upon the temperature of 
 the climate — evident adaptations to a colder 
 and more changeable climate, which at 
 that time must have led them easily to 
 domination in the animal world. — The 
 
58 
 
 THE CLIMATES 
 
 intenser activity of the vital interchange 
 of matter in the homoeothermal animals, 
 necessary to maintain the high and con- 
 stant temperature of their bodies, was 
 the immediate origin of the circulation 
 of the blood and of the respiration, as 
 well as of the organs which effect a faster 
 and more thorough utilisation of food.* 
 In connection with this we observe that 
 the teeth of the mammals, which during 
 the whole of the Mesozoic period obtained 
 no higher degree of perfection than the 
 multitubercular type, already existing in 
 the Triassic period, from the beginning 
 
 * Compare: R. Baume, Versuch einer Entwickelungsge- 
 schichte, des Gebisses. Leipzig, 1882, p. 284. 
 
 Mammals have on the average nearly a tenfold intenser 
 consumption of matter than reptiles and amphibians at 
 20 0 air temperature. At i° air temperature this is in the 
 cold-blooded vertebrates nearly zero, and only at 36° is it 
 equal to that of the homoeothermal animals. (Compare : L. 
 Hermann, Lehrhuch der Physiologie. 9te Auflage. Berlin, 
 1889, pp. 215 and 212). 
 
OF THE GEOLOGICAL PAST. 
 
 59 
 
 of the Tertiary period, develop rapidly, 
 starting from the primitive Mesozoic trituber- 
 cular type. — Again, the constantly intenser 
 activity of the vital interchange of matter 
 rendered possible a uniform production 
 of a greater amount of external work, 
 and facilitated the building up of the body 
 whenever and wherever it was necessary. 
 Thus the original requirement for the pro- 
 duction of more heat caused the mammals 
 and birds to attain higher functions and 
 acquire more perfect forms than could be 
 obtained by the poikilothermal reptiles ; 
 and even the high development of the 
 brain in the mammal class may be con- 
 sidered as the result of that diminution 
 of external heat which took place during 
 the Cainozoic period.* 
 
 * Lartet and Marsh, it is well known, have really proved 
 a gradual progress of development in the brain of mammals 
 during the Tertiary period. 
 
6o 
 
 THE CLIMATES 
 
 The Pliocene climate was but slightly 
 warmer than the present, and at the commence- 
 ment of the Pleistocene period the climatic 
 condition of the earth was the same as 
 that which prevails at present. Still later, 
 a general cooling took place over the 
 whole earth, the temperature falling below the 
 present, and resulting in a general lowering 
 of the snow line, which again caused 
 enormous glaciers to descend from Scan- 
 dinavia, Finland, and the Russian Baltic 
 provinces, over a great part of Europe, 
 as far as England, the Netherlands, Central 
 Germany, and far in the South of Russia. 
 A similar continental ice sheet covered 
 North America as far as the 39th degree 
 N.L. (the latitude of Lisbon in Europe), 
 Great Britain, the Alps, the other high 
 mountain ranges of Central Europe, the 
 Pyrenees and the Apennines, the Caucasus, 
 the Atlas in North Africa and even, 
 
OF THE GEOLOGICAL PAST. 
 
 6l 
 
 though in less degree, the high mountains 
 in the dry regions of Northern Asia : also 
 the Himalayas, the Karakorum and 
 Tien Shan mountains. In the southern 
 hemisphere, Patagonia, the Andes of Chili, 
 perhaps also the higher mountains 
 of South Africa, the Australian Alps 
 and especially New Zealand, also the 
 Kerguelen, the South Georgia and 
 Falkland Islands had their own gigantic 
 system of glaciers, partly extensions 
 of the present ones, and partly existing 
 on the higher mountains which are not 
 ice-covered at present. All these regions 
 are still covered with mineral matter 
 transported by glaciers. Nearer to the 
 equator traces have also been found of 
 Pleistocene glaciers in the Sierra Nevada 
 de Santa Marta (at n° N.L.) and the 
 Andes of Cocui (4-|- 0 to 7-J 0 N.L.) in 
 Columbia, and in the Sierra Nevada de 
 
62 
 
 THE CLIMATES 
 
 Merida (8° N.L.) in Venezuela. * These 
 occurrences of Pleistocene Glacial pheno- 
 mena near the equator are one of the best 
 evidences that the glaciation of the north- 
 ern and southern hemispheres took place 
 simultaneously, so that its general character 
 can no more be doubted. *j* 
 
 This general depression of the snow 
 line was accompanied by a generally estab- 
 
 *W. Sievers, Ueber Schotterterrassen, Seen und Eiszeit 
 im nordlichen Siidamerika. Geogr. Abhandl. herausgegeben 
 von A. Penck. Wien 1887. Yol. 2. — A. Hettner, Reisen 
 in den Kolumbianischen Anden. Leipzig 1888, p. 319, and: 
 Die Kordillere von Bogota. Petermann’s Mitteilungen. 
 Erganzungsheft No. 124. 1892, p. 75. 
 
 f About this general and simultaneous prevalence of the 
 Glacial phenomenon see: A. Penck, La periode glaciaire 
 dans les Pyrenees. Bull. Soc. d’hist. nat. Toulouse. 1885. 
 Vol. 19, p. 107. Compare also his compendious description 
 of the entire Glacial phenomenon according to the present 
 state of our knowledge : Die grosse Eiszeit in : Himmel und 
 Erde. 4. Jahrgang. 1891, pp. 1 and 74. 
 
 Beyond the above mentioned pleistocene glaciations in the 
 tropics, the following ones have been found, which I mention 
 
OF THE GEOLOGICAL PAST. 63 
 
 lished increase of lakes without effluent.* 
 As the dampness of the climate is a 
 still more important factor in the formation 
 of glaciers than a low temperature, it has 
 been suggested that during the Glacial 
 epoch the temperature might not neces- 
 sarily have been lower, but might even 
 have been higher than at present, j- Yet 
 
 here on account of the importance of this subject’s bearing; 
 in the Andes of Ecuador (El Altar) near the Equator, in the 
 Ilimani and the Andes of Bolivia between 1 5 0 and 1 8 J / 2 0 S.L. 
 in the Anahuac of Mexico as far as 17 0 N.L. and in the 
 Namuli mountains of East- Africa at 16 0 S.L. The glaciation 
 of the Sierra do Mar in Brazil asserted by Agassiz and 
 Hartt is according to J. C. Branner (A. R. Wallace, 
 Darwinism, London, 1890, p. 370) decidedly problematical. — 
 Compare also different writings of Penck, especially his latest 
 essay in: Himmel und Erde, and: Map. No. 5 of the geo- 
 logical part of the new edition of Berghaus’ Physikalischer 
 Atlas. 
 
 *E. Bruckner, Klimaschwankungen seit 1700, nebst Be- 
 merkungen fiber die Klimaschwankungen der Diluvialzeit. 
 Wien 1890. Cap. x. 
 
 |J. D. Whitney, The climatic changes of later geological 
 
64 
 
 TPIE CLIMATES 
 
 not only have serious objections been raised 
 by meteorologists against this suggestion,* 
 but also the palaeontological facts of the 
 Glacial epoch are in no wise in harmony 
 therewith. Not only was there a depression 
 of the snow line, but also of the forest 
 line, which certainly is only dependent 
 upon the temperature. The entire flora 
 and fauna of the glaciated regions belong 
 to a cool climate, f and cannot in the 
 least be compared to the luxuriant life of 
 Patagonia and New Zealand where, favoured 
 
 times. Mem. of the Museum of Compar. Zoology at Harvard 
 College. Vol. 7. No. 2. Cambridge, U. S. A. 1882. 
 
 * A. Woeikof, Die Klimate der Erde. Part. 1, p. 102 sqq. 
 
 -j-A recent treatise on the distribution of Arctic plants 
 during the Glacial epoch in Central Europe, by A. G. Nat- 
 horst, appeared in: Nature, Vol. 45, 1892, pp. 273 — 275: 
 Fresh evidence concerning the distribution of Arctic plants 
 during the Glacial Epoch. — The Arctic mammalia which are 
 known at present from the Diluvian beds of Central Europe 
 are mentioned by A. Nehring, Ueber Tundren und Steppen 
 der Jetzt- und Vorzeit. Berlin 1890, p. 167. 
 
OF THE GEOLOGICAL PAST. 65 
 
 by a relatively warm, but especially uniform 
 and damp climate, a subtropical vegeta- 
 tion can exist even in between the glaciers. 
 
 In calculating approximately the amount 
 of melting (ablation) at the known snow 
 line of the Glacial epoch with the distribu- 
 tion of heat, supposed to have been the same 
 as now, this is found to be three or four 
 times as considerable as it is at the present 
 snow line. During the Glacial epoch the 
 precipitation ought, therefore, to have been 
 three or four times as much as it is at 
 present ; which is certainly impossible. * 
 The former supposition that over Europe 
 Arctic cold then prevailed has long been 
 given up, still everything indicates that the 
 temperature must have been several degrees 
 less than the present, f Judging from the 
 
 *A. Penck, Die grosse Eiszeit, l.c., p. 13. 
 fThis conclusion has been arrived at principally through 
 climatic investigations of A. Woeikof, Ueber die klimatischen 
 
 5 
 
66 
 
 THE CLIMATES 
 
 character of the flora and fauna, and from 
 the extension of the ice, we may infer that 
 the difference from the present condition 
 must, at temperate latitudes, have been 
 less than 5 0 C. Penck succeeded in com- 
 puting approximately the height of the 
 pleistocene snow line on a number of moun- 
 tains, and found it to have been on the 
 average about 1000 metres (from 500 
 metres to 1300 metres) lower than the 
 present, which corresponds with a general 
 reduction of the earth’s temperature of 
 5 0 C, * He also calculated that during the 
 Glacial epoch such a considerable quantity 
 of water was withdrawn from the ocean, 
 that its mean level must have been about 
 100 metres lower. “A general reduction 
 
 Verhaltnisse der Eiszeiten sonst und jetzt. Verhandl. d. 
 Gesellsch. fur Erdkunde. Berlin 1880. Vol. 7, p. 15 1 sqq. 
 
 *A. Penck, Die Eiszeit in den Pyrenaen. Mitteilg. d. Ver. 
 f. Erdkunde. Leipzig 1884, p. 209. 
 
OF THE GEOLOGICAL PAST. 
 
 6? 
 
 of temperature of 4 0 , 5 C. — and for a 
 climate damper than the present, of even 
 less — suffices to account for the whole 
 Glacial phenomenon. ”* 
 
 The fauna of the North Sea had an 
 arctic character. Torell stated that the 
 marine mollusca and the marine mammals, 
 which are found in the Yoldia clays of 
 the Cattegat and the North Sea, indicate 
 temperatures which prevail at present in 
 the Karian Sea.f In all probability, there- 
 fore, the temperature of the sea water in 
 that part of Europe was, during the coldest 
 period, 5 0 to io° C. lower than the 
 present; which is quite natural when we 
 take into account that huge masses of 
 northern inland ice descended into those 
 seas. The Mediterranean Sea at that 
 
 * A. Penck, Die grosse Eiszeit, p. 77 — 79. 
 
 -j*0. Torell in: Zeitschrift der Deutschen Geologischen 
 Gesellschaft. 1888. Vol. 40, p. 250 sqq. 
 
68 
 
 THE CLIMATES 
 
 period, also contained many organisms 
 which now exist in the North Sea. To 
 judge from the organisms, the difference 
 of temperature, at least for the sea cli- 
 mate, must have decreased towards the 
 equator. 
 
 This cold, which prevailed over the 
 whole earth, did not consecutively last 
 up to the beginning of our time. In 
 Europe and North America, where the 
 Glacial phenomena have been longest known 
 and most fully investigated, an intermediate 
 bed, which shows no trace of ice agency 
 and contains remains of plants and animals 
 indicating a climate no cooler than the pre- 
 sent, has been found in many places be- 
 tween Glacial formations. These Interglacial 
 formations prove that during the Glacial 
 epoch a recurrence of the temperature 
 which prevailed at the beginning of the 
 Diluvian age intervened, during which, 
 
OF THE GEOLOGICAL PAST. 
 
 69 
 
 the great ice masses dissolved and a 
 flora and a fauna existed similar to 
 that of the Preglacial and Postglacial (the 
 present) epoch but indicative of a drier 
 climate during the acme of the epoch 
 (Steppe periods). * A second, older, shor- 
 ter and less conspicuous Interglacial epoch 
 has been found in Europe, which, never- 
 theless, must have lasted much longer than 
 the Postglacial epoch, in which we live. | 
 
 * Compare : F. von Richthofen, China. Vol. 2, p. 348, 
 and: Ueber die Bildung des Loss. Verhandl. der KK. Geol. 
 Reichsanstalt in Wien. 1878, pp. 289 — 296, and different 
 treatise on Steppenfaunae by A. Nehring, chiefly his recent 
 comprehensive treatise : Ueber Tundren und Steppen der Jetzt- 
 und Vorzeit. Berlin 1890. 
 
 -j*A. Penck, Zur Vergletscherung der Deutschen Alpen. 
 Leopoldina. Vol. 21. Halle 1885, pp. 105, 129 and 145. — 
 E. Bruckner, Die Vergletscherung des Salzachgebietes, etc. 
 Geogr. Abhand. edited by A. Penck. Wien 1886; I, p. 133, 
 and : Die Eiszeit in den Alpen. Mitteil. Geogr. Gesellsch. 
 Hamburg 1887 — 1888, pp. 10 — 23, and further his: Kli- 
 maschwankungen. Cap. x. 
 
70 
 
 THE CLIMATES 
 
 The shorter duration of the last mentioned 
 epoch is as apparent from the relative 
 thickness of the Interglacial and Postglacial 
 Weathering-Loam as from the Interlacustre 
 and Postlacustre deposits in the North Ame- 
 rican lake basins of the Diluvial period 
 investigated by Gilbert and Russell. The 
 penultimate Ice Age was of longer duration 
 than the last one, and the oldest known was 
 the shortest. 
 
 Thus the Glacial phenomenon consists 
 of a general cooling of only a few degrees, 
 a cold which differed still less from the 
 present temperature than the higher tem- 
 perature of the Pretertiary period ; it 
 occurred certainly twice and probably 
 three times. During the long intervals 
 which separate these cold epochs the 
 same climatic condition recurred which 
 obtained earlier and later and still pre- 
 vails now. These cold epochs must, there- 
 
OF THE GEOLOGICAL PAST. 
 
 71 
 
 fore, be considered as interruptions of 
 the present condition, which set in as early 
 as the beginning of the Pleistocene period, 
 immediately after the Tertiary period. 
 
PART II. 
 
 Changes in the Solar Heat as the 
 Agency by which the Geological 
 Climatic Changes were 
 brought about. 
 
 Considerable changes of the solar radiation sufficient to 
 account for the climatic changes. — The postulated changes 
 of solar radiation have actually taken place. — The solar 
 heat arises most probably from contraction. Seeming in- 
 consistency with geological and biological data. — Further 
 circumstances in favour of the view that the geological changes 
 of climate are to be explained by the history of the sun’s 
 evolution. 
 
 Considerable Changes of the Solar Radiation 
 sufficient to account for the Climatic Changes. 
 
 In the preceding Part the general features 
 of the great changes in the heat on the 
 
THE CLIMATES OF THE GEOLOGICAL PAST. 73 
 
 surface of the earth, according to the palaeon- 
 tological and geological data, were discussed. 
 What can have been the cause of these 
 changes? As it has now been sufficiently 
 proved that tellural agencies (internal heat of 
 the earth, a possible different mixing of the 
 atmospheric gases, changes in the division 
 of land and sea),* if they prevailed at all, 
 
 * The belief is still too common that ages have been, during 
 which heat rising from the interior of the earth could have 
 contributed in a high degree to the development of life on the 
 earth; though as early as 1862 Sir William Thomson (now 
 Lord Kelvin) proved “ that the general climate cannot be 
 “ sensibly affected by conducted heat at any time more than 
 “ 10,000 years after the commencement of superficial solidi- 
 fication.” (On the secular cooling of the earth. Trans. Roy. 
 Soc. Edinburgh. Vol. 23. § 17, also published in his: Mathe- 
 matical and Physical Papers. Vol. 3. London 1890, p. 305). 
 That solidification must have commenced more than 20,000,000 
 years ago (Mathemat. and Phys. Papers. Vol. 3, p. 300). 
 
 I need not dwell here upon the astronomical hypotheses 
 of Croll, Ball, and others for explaining the geological 
 climatic changes. These, as it seems to me, have been con- 
 demned sufficiently, though they still find many adherents, 
 
74 
 
 THE CLIMATES 
 
 could never have sensibly modified the 
 general heating of the earth during the 
 time it has been inhabited; and it being 
 further proved that the more uniform 
 distribution of heat up to the Tertiary 
 period was such that, though places of 
 high latitudes were favoured by it, this 
 
 chiefly in England. (Compare: A. Woeikof, Examination 
 of Dr. Croll’s Hypotheses of geological climates. American 
 Journal of Science. 1886. (3) Vol. 31, p. 161, and: Philosoph. 
 Magazine. 1886, p. 233 sqq.) Yet it must be mentioned 
 that, as Neumayr (Erdgeschichte Vol. 2, p. 649), draws 
 attention to, the climatic conditions of the Diluvian period, 
 on which these explanations are based, form but an exceedingly 
 small part of the climatic changes which occurred during 
 untold millions of years. “Notwithstanding all study it is, 
 “therefore, not possible to form a just opinion from this 
 “ exceedingly limited space of time. We must first try to 
 “ know the climatic conditions of the former periods” .... 
 And, certainly, neither the development of the organic world, 
 nor its distribution in the former ages, agree with the as- 
 sumption of winter cold then already existing and recurrent 
 glacial and interglacial periods, for the assumption of which 
 geological data likewise do not plead. 
 
OF THE GEOLOGICAL PAST'. 
 
 75 
 
 was not at the expense of the equatorial 
 regions, it is obvious that the total amount 
 of heat received by the earth must have 
 been greater than at present. In fact it must 
 have been considerably greater ; for in 
 January the earth receives 6^0/0 more 
 heat from the sun than in July and this 
 difference not only nowhere produces any 
 noticeable effect, but its general tempera- 
 ture in July is even higher than in Janu- 
 ary.* The influence of the larger share 
 of solar heat, which the earth receives in 
 winter time, does not appear to be felt 
 through the thick atmospheric mantle which 
 is so changeable in its motion, the amount 
 of vapour it holds and its clouds. The 
 unequal distribution of land and sea in the 
 northern and southern hemispheres — an 
 
 * R. Spitaler, Die Warmevertheilung auf der Erdoberflache. 
 Denkschr. Math. Nat. Kl. K. Akad. der Wissensch. Wien 
 1886. Vol. 51, p. 1. 
 
7b 
 
 THE CLIMATES 
 
 unimportant factor in the general diffusion 
 of heat over the globe, as has been proved 
 by geological experience* — influences the 
 general temperature more than this increase 
 of one fifteenth of the solar heat. 
 
 In the, geologically speaking, recent 
 period, during which the present diffusion 
 of heat prevailed, it has twice happened 
 that the earth received a considerably 
 smaller quantity of heat than at present, 
 as otherwise there would not have been 
 a marked influence upon the climates. 
 
 It now seems to be completely proved 
 that no other source of heat than the sun 
 can ever have exercised an appreciable in- 
 fluence either on the meteorological condition 
 of the earth, or on the climates. To 
 any very considerable changes of the solar 
 heat we must, therefore, look for the cause 
 
 * Chiefly by the valuable investigation made by Neumayr, 
 Die geographische Verbreitung der Juraformation. l.c. 
 
OF THE GEOLOGICAL PAST. 
 
 77 
 
 of the geological changes of climate. But 
 the question then arises whether, assuming 
 a considerable diminution or increase of 
 solar radiation, the diffusion of heat 
 over the surface of the earth would be 
 like what we know that it must have 
 been from the geographic distribution of 
 organisms ! 
 
 This must really have been the case, as 
 is proved in the first instance by the present 
 diffusion of heat. Comparing the various 
 temperatures for different latitudes, as com* 
 puted by Zenker* from the solar radiation, 
 with those found by SpiTALERf from obser- 
 vation (Table xxii of Zenker), a consider- 
 able displacement of heat from the equator 
 towards the north pole becomes apparent. 
 
 *W. Zenker, Die Vertheilung der Warme auf der Erdo- 
 berflache. Berlin, 1888. 
 
 + R. Spitaler, Die Warmevertheilung auf der Erdober- 
 flache, l.c. 
 
78 
 
 THE CLIMATES 
 
 Between 50 0 and 70 0 northern latitude a 
 great excess of heat is found, which cor- 
 responds with a deficiency in the equatorial 
 regions. In the transition zone, from io° 
 to 50 0 northern latitude, that excess is 
 only very slight. Its origin must be 
 found, as may be seen from the maps of 
 thermal isonomals of Spitaler,* in the 
 warm ocean currents and in the predomi- 
 nating winds. Thus the yearly tempera- 
 ture in the Atlantic Ocean north-west of 
 Norway is about 1 2 0 C., and in January even 
 24 0 C. higher than it ought to be theo- 
 retically, according to the latitude. This 
 same influence is still very marked 
 in the temperature of Spitzbergen and 
 even of Greenland and Nowaya Zemlja. 
 Comparing the land temperatures at the 
 equator, as computed by Zenker and also 
 
 * Ibid. — Compare: Petermann’s Mitteilungen 1887. Taf. 
 20 and 1889. Taf. 17 and 18. 
 
OF THE GEOLOGICAL PAST. 
 
 79 
 
 by Forbes and Spitaler,* with that which 
 has been actually observed, a great difference 
 is apparent, the latter being io and more 
 degrees Celsius lower ; the observed sea 
 climate of the equator, on the contrary, 
 being nearly the same or even a little 
 warmer than the computed one. Hence 
 we may conclude that not a small amount 
 of the excess of heat in high latitudes is 
 due to the heat received by the land in 
 equatorial regions. The southern hemi- 
 sphere is cooler than it ought to be accord- 
 ing to calculation, which must ultimately 
 be attributed to the fact that it consists 
 mainly of sea. j* As it is known, however, 
 that the northern hemisphere had even in 
 
 *W. Zenker, l.c. Tab. xxiii. J. D. Forbes, Inquiries 
 about terrestrial temperature. Trans. Roy. Soc. Edinburgh. 
 Vol. 22. — R. Spitaler, Die Warmevertheilung auf der 
 Erdoberflache. l.c. 
 
 f Compare: Chapters VI and XVI of Vol. I of: Woeikof, 
 Klimate der Erde. 
 
8o 
 
 THE CLIMATES 
 
 early ages a relatively large proportion of 
 land, and as we judge of the temperature 
 of early climates mainly by land organisms, 
 we may safely deduce from the results 
 obtained by Zenker for this hemisphere 
 in its present condition its climatic con- 
 ditions in former periods. 
 
 Changes in the solar radiation must, 
 according to Zenker, have three times as 
 strong an effect on the computed (solar) 
 land temperature of the equatorial regions 
 as on temperate and polar latitudes, the 
 solar land temperatures increasing in pro- 
 portion to the augmentation of the yearly 
 solar radiation. The excess of heat at 
 high latitudes must, therefore, increase with 
 an increase of solar radiation and must 
 have embraced a broader zone than at 
 present, owing, as now, to the heat re- 
 ceived by the sea at the equator — which 
 heat, as will be more fully explained here- 
 
OF THE GEOLOGICAL PAST. 
 
 8l 
 
 after, would then be yet more easily trans- 
 ported because of the more rapid circulation 
 of the atmosphere. 
 
 Owing to the extensive area of the tropical 
 belt, 40 0 /o or two fifths of the total area 
 of the earth’s surface — which embraces an 
 area almost as large as the two temperate 
 belts, and four times as large as the two 
 polar caps — and the large amount of heat 
 received, even small changes in its tem- 
 perature must exercise a great influence on 
 the climate of high latitudes. 
 
 This influence is mainly exercised by 
 the agency of ocean currents, caused by 
 the prevailing winds (which themselves 
 probably play an important direct part in 
 the distribution of heat). But even though 
 the sea area within the tropical belt should 
 have been much smaller than the present 
 (three fourths of the total area), the influ- 
 ence of the equatorial heat on the higher 
 
 6 
 
82 
 
 THE CLIMATES 
 
 latitudes would not necessarily have been 
 less, as these latitudes profit so greatly 
 by the heat received by the land at the 
 equator. The direct result of an increase 
 of land area at the equator would, 
 therefore, be an increase of the excess 
 of heat at high latitudes, though this 
 increase would have been compensated 
 by the decreased quantity of heat trans- 
 porting water. 
 
 Broad communications between the seas 
 at high latitudes and the tropical seas, 
 and connections, such that the warm water 
 could flow through in the direction indi- 
 cated by the predominating winds, must, 
 however, have been necessary conditions 
 for the influence described above. We 
 know with certainty that in the Carboni- 
 ferous, the Jurassic, the Cretaceous and 
 the Tertiary periods, of which the arctic 
 floras are known, such broad connec- 
 
OF THE GEOLOGICAL PAST. 
 
 83 
 
 tions, even much broader than the 
 present, existed between the arctic seas 
 and those near the equator or in the 
 tropics. 
 
 An increase of slow radiation must in 
 the first place have directly raised the 
 temperatures of temperate and high latitudes, 
 chiefly in favour of the land temperature — 
 but a far greater influence must have 
 been exercised indirectly by the heat 
 transported from the tropics by warm 
 ocean currents by which the sea climate 
 would chiefly be benefited. 
 
 Diminution of the sun’s heat would, on 
 the contrary, have had the effect, that in 
 high latitudes the indirect cooling of the 
 sea was greater than the direct one which 
 had mainly affected the land. 
 
 At an age in which the surface of the 
 southern hemisphere consisted, as at present, 
 mainly of water, the changes of the heat 
 
8 4 
 
 THE CLIMATES 
 
 received there must have been partially 
 felt in the northern hemisphere: in the 
 same manner as at present a great deal 
 of the warm water of the southern hemi- 
 sphere flows into the northern hemisphere. 
 The distribution of land and sea was, at 
 least during the Pleistocene period, 
 very probably the same as it is now, 
 and thus the smaller supply of warm 
 water to the northern hemisphere must 
 have been a cause of refrigeration of that 
 hemisphere. 
 
 A much warmer sea washing the shores 
 of a still warmer continent, and a more 
 considerable exchange of heat in the 
 direction of the meridians, caused by the 
 stronger general system of winds — this 
 was, therefore, the climatic condition in 
 higher latitudes under the reign of a hotter 
 sun — a cool sea, more especially in 
 the northern hemisphere, together with a 
 
OF THE GEOLOGICAL PAST. 85 
 
 slightly cooler continent, must have been 
 the effect of a less hot sun. A similar 
 condition in which the seas are cooler than 
 the land obtains nowadays in Patagonia 
 and New Zealand, where glaciation is 
 stronger now, than anywhere else at a 
 temperate latitude. Such a condition is the 
 most favourable for glaciation and we may 
 safely conclude, with J. D. Forbes and 
 Woeikof,* that by a considerable cooling 
 of the seas between 40 0 to 6o° N.L. many 
 mountains in Europe and in the western 
 part of North America would be covered 
 by glaciers and that the existing glaciers 
 would increase. 
 
 It is quite possible that in the tropics, 
 where a greater dampness must have 
 prevailed during the Pleistocene period, 
 even slight diminutions of the land tem- 
 perature, which probably have taken 
 
 *A Woeikof, Die Klimate der Erde. I. p. 104. 
 
86 
 
 THE CLIMATES 
 
 place, caused a considerable increase of 
 the present glaciers and originated new 
 ones. 
 
 There is now every reason for assuming 
 that actually, under the influence of a 
 hotter or less hot sun, the ocean cur- 
 rents, which can exercise in the diffusion 
 of heat over the surface of the earth so 
 great an influence,* and the prevailing 
 winds, which are the cause of the sea 
 currents, and apparently also exercise a 
 strong direct influence, j have had, as a 
 rule the same direction as now, and that 
 
 *The works of Croll are still fully deserving of attention 
 in this respect, chiefly : Climate and Time in their Geological 
 Relations. London 1875, and his earlier essay: On Ocean 
 Currents in Relation to the Distribution of Heat over the 
 Globe. Philosoph. Magazine, 1870. 
 
 fH. von Helmholtz has in his papers: Ueber atmos- 
 pharische Bewegungen. Sitzungsber. der K. Pr. Akad. d. 
 Wiss. Berlin 1888, and: Meteor Zeitschr. 1888, Vol. 5, pp. 
 329 — 340, pointed out that by the displacement of the air 
 masses, from the equator in a northerly and southerly direction, 
 
OF THE GEOLOGICAL PAST. 87 
 
 also the force of these winds varies 
 considerably according as the sun’s heat 
 in more or less powerful. More and 
 more, nowadays meteorologists recognise 
 the fact that a grand system of circulation 
 prevails in the atmosphere of the earth, 
 between the equator and the poles, of 
 which the predominating winds form part. 
 Chiefly by the admirable theoretical in- 
 vestigations of Ferrel and Werner Sie- 
 mens (who, starting from entirely different 
 bases, arrived at the same main con- 
 clusions), * as well as by the observation 
 of atmospheric pressure and temperatures 
 
 an enormous amount of kinetic energy is used, which is 
 transformed into heat. 
 
 *As early as i860 W. Ferrel proclaimed the existence 
 of a general circulation. More within the reach of everyone 
 than his earlier publications is his : Popular Treatise on the 
 Winds. London 1890. 
 
 Werner Siemens, Ueber die Erhaltung der Kraft im 
 Luftmeere der Erde. Sitzungsber. d. K. Pr. Akad. d. Wiss. 
 Berlin 1886. Vol. 13, p. 261. 
 
88 
 
 THE CLIMATES 
 
 at mountain stations, and their interpreta- 
 tion by Hann, it is now a recognised fact 
 that a general circulation prevails in the 
 atmosphere, dependent either upon the 
 rising and then poleward directed force of 
 the heated air at the equator (Siemens) or 
 from the constant difference of temperature 
 between the poles and the equator, 
 namely the thermal gradient (Ferrel). This 
 general wind system consists on the sur- 
 face of the earth of two large whirls, one 
 in either hemisphere, moving according to 
 the law of Buys Ballot, and an equatorial 
 wind belt moving in an opposite direction. 
 It produces at the surface of the earth at 
 temperate and higher latitudes, from the 
 35th degree, in the northern hemisphere 
 south-westerly winds and in the southern 
 hemisphere north-westerly winds (westerly 
 winds), the directions of the winds near 
 the equator being north-east in the northern 
 
OF THE GEOLOGICAL PAST. 
 
 89 
 
 hemisphere and south-east in the southern 
 hemisphere (trade winds). At the 35th 
 degree of latitude (calm belts of Ross) there 
 is an increase in the atmospheric pressure, 
 which is caused, either by the “ centrifugal 
 force” (Ferrel) or by the forcing up of 
 the poleward upper current in consequence 
 of the narrowing of the bed of the current 
 (Siemens). According to Siemens — and 
 his theoretical investigations have been in- 
 ductively affirmed by Hann* — cyclones also 
 owe their origin to this general circulation; 
 the current, which, by the narrowing of 
 
 * See the most recent paper of this eminent meteorologist : 
 J. Hann, Das Luftdruck-Maximum von November 1889 in 
 Mittel-Europa, nebst Bemerkungen iiber die Barometer-Maxima 
 im Allgemeinen. Denkschr. Math. Nat. Kl. d. K. Akad. 
 d. Wiss. Wien 1890. Vol. 57, pp. 401 — 424, and: Studien 
 iiber die Bedingungen von Luftdruck und Temperatur auf 
 der Spitze des Sonnblick, nebst Bemerkungen iiber ihre 
 Wichtigkeit fur die Theorie der Cyclonen und Anticyclonen. 
 Ibid. 1891. Vol. 58. 
 
90 
 
 THE CLIMATES 
 
 its bed, is continually retarded on its way 
 towards the pole, and which by the earth’s 
 rotation is continually deviated towards 
 the last, draws the lower atmospheric 
 layers along by internal friction, thus pro- 
 ducing local minima. 
 
 This general circulation of the atmosphere 
 will intensify increases with the differ- 
 ence between the heating of the equator 
 and the poles — as is the case in every 
 augmentation of solar radiation — and will 
 diminish with a decrease of that difference — 
 as in every diminution of solar radiation. 
 Moreover, in the first case the meridian 
 component of the wind’s direction (that is, 
 the one which is dependent upon the 
 amount of heating) will be predominant, in 
 the latter case the parallel one (that which 
 corresponds with the earth’s rotation). 
 Thus, at a time when the earth receives 
 more heat, the winds on the polar sides 
 
OF THE GEOLOGICAL PAST. 
 
 91 
 
 of the 35th degree N. and S. latitude 
 not only increase in force, but have 
 also a more northerly direction; while 
 at a time of less heating of the earth 
 their direction must be still more easterly 
 than under the present conditions. Further, 
 the ocean currents, which owe their origin 
 to this general wind-system, are, with regard 
 to their power, velocity and direction, 
 subject to the same changes and will 
 transport, in the first case much more, in 
 the latter case much less heat from the 
 equator towards the poles. As the main 
 directions of the oceans in the Diluvial 
 period were apparently the same as at 
 present, then the continents must have had 
 in the latter case — with more easterly 
 directed winds — also a damper climate 
 (corresponding with Glacial periods), and 
 in the periods of moderately increased solar 
 radiation a drier climate with winds of a 
 
92 
 
 THE CLIMATES 
 
 more northerly direction (Interglacial Steppe 
 periods). But at the time of a greater 
 increase of radiation intensity, the result 
 of the prevailing winds setting in a more 
 meridianal direction would certainly not 
 equalize their greater increase of force 
 and quantity of vapour — at least where 
 predominating influences otherwise acting 
 did not exist, as seems to have been 
 the case in Europe during the Eocene 
 period.* 
 
 The influence of the reduced force of 
 ocean currents (and of predominating winds) 
 during the Ice age is apparent from 
 the fact that, during that period, 
 Europe lost the climatic advantage which 
 it enjoys at present over the east 
 coast of North America. The British 
 
 ♦According to de Saporta the flora of the European 
 Eocene was that of a warm dry country with little rainfall. 
 
OF THE GEOLOGICAL PAST. 
 
 93 
 
 seas in particular suffered greatly in this 
 respect.* 
 
 Further we observe that the system 
 of westerly winds increases in force and 
 sets in at lower latitudes during the winter, 
 when the difference between the temper- 
 ature at the equator and at the pole of 
 the relative hemisphere is greater than in 
 summer. This temporary increase of the 
 force of the predominating winds can have 
 no perceptible effect on the ocean cur- 
 rents, which are the result of the entire 
 work performed for centuries by those 
 winds. Their moment of motion is much 
 smaller than that of the ocean currents, f 
 and their immediate effect, therefore, almost 
 
 *Sir J. W. Dawson, The Story of the Earth and Man. 
 io*h Edition. London 1890, p. 273. 
 
 |G. von Boguslawski and O. Krummel, Handbuch der 
 Oceanographie. Stuttgart 1887. Vol. 2, p. 351. The proportion 
 of the moment of motion of .air currents to that of ocean 
 currents is according to Krummel as 1 : 123. 
 
94 
 
 THE CLIMATES 
 
 imperceptibly small ; the stationary state of 
 motion of the ocean, corresponding to the 
 average velocity of the winds, has obtained 
 for thousands of years, and the winds have 
 only now to make good the loss caused 
 by friction, to which their moment of 
 motion certainly suffices.* For this reason 
 the stronger circulation of the air during 
 the winter of either hemisphere cannot 
 serve to raise its temperature perceptibly, 
 for by the smaller specific heat of the air, 
 in comparison to that of land and water, 
 it will not reach temperate latitudes before 
 being entirely cooled in its long journey 
 from the equator, and this winterly strength- 
 ening of the movement of the air is not 
 yet sufficiently strong to exercise a con- 
 siderable influence by its kinetic energy 
 becoming heat. 
 
 Ibid., p. 350. Conclusion of Hann. 
 
OF THE GEOLOGICAL PAST. 
 
 95 
 
 This direct influence of the winds 
 towards an equalisation of the differences be- 
 tween the temperature at the equator and 
 at the poles must certainly have been greater 
 during the time of a stronger circulation,* 
 but even at that time it must still have 
 been inferior to the influence exercised by 
 the ocean currents whose specific heat is so 
 much greater. Assuredly this influence of 
 the winds must — then, as now — have con- 
 tributed more towards equalising the diffu- 
 sion of heat over short distances, between 
 neighbouring seas and lands. 
 
 That the direct influence of changes in 
 the solar radiation — though according to 
 Zenker three times as great near the equa- 
 tor as in temperate and polar latitudes — 
 
 * According to the investigation of H. von Helmholtz 
 (Ueber atmospharische Bewegungen. 11. cc.) the direct influence 
 of a considerable increase of the general circulation must have 
 been very important. 
 
96 
 
 THE CLIMATES 
 
 does not, actually, in the first place affect 
 the temperature at the equator, but the 
 general circulation of the atmosphere, has 
 been proved by the relation which exists 
 between the eleven year sun-spot periods 
 and meteorological phenomena. During the 
 maxima of sun-spot periods the solar radi- 
 ation is certainly largest. This is proved, 
 in addition to spectroscopic observations, * 
 in the first place by the fact that, according to 
 H. F. Blanford, the atmospheric pressure 
 then decreases at the earth's surface between 
 the tropics (South Eastern Asia) and 
 
 * The surface of the photosphere covered with faculse, which 
 displays a noticeable increase of the continuous part of the 
 spectrum, is much greater than that of sun-spots (generally 
 increasing and decreasing parallelly with the faculse), and 
 these spots, according to their spectrum and to direct measure- 
 ments, emit less heat than the main photosphere. Further, 
 the spectrum of a sun-spot in the time of minimum seems 
 to indicate a lower temperature than that of a sun-spot of 
 maximum period (S. J. Perty and A. L. Cortie, Monthly 
 Notices Roy. Ast. Soc., 49, 1889, p. 410). 
 
OF THE GEOLOGICAL PAST. 
 
 97 
 
 increases at higher latitudes (Western Si- 
 beria and Russia in winter), apparently 
 owing- to an increase of the difference in 
 heating between the equator and the poles, 
 which must be accompanied in the upper 
 regions of the atmosphere by an increase 
 of the surplus of atmospheric pressure at 
 the equator, which, again, causes an in- 
 crease of the general wind system ; * in 
 the second place by the fact that, during 
 the maxima of sun-spot periods, cyclones are 
 most numerous in the tropics and, on the 
 contrary, are scarce during the minima of 
 sun-spot periods (C. Meldrum, A. Poey) ; 
 and in the third place by the fact that 
 probably the rainfall is most abundant 
 
 *E. Bruckner also pointed out, that for the 35 year 
 periods of climatic changes — which he attributes likewise to 
 oscillations in the intensity of solar radiation — the difference 
 between the atmospheric pressure at the equator and at sub- 
 polar latitudes increases in warm periods, and decreases in 
 cool periods. 
 
 7 
 
98 
 
 THE CLIMATES 
 
 during the maxima of sun-spot periods, which 
 points to an increase of evaporation and 
 an increased motion in the atmosphere. 
 Still, the temperature of the air near the 
 equator is generally lowest in maxima of 
 sun-spot periods and highest in minima 
 of sun-spot periods (W. Koppen, H. F. 
 Blanford.) 
 
 All these phenomena make it apparent 
 that the atmosphere operates as a very 
 active regulator in the diffusion of heat 
 over the surface of the earth, and that this 
 activity is decreasing or increasing with 
 the amount of heat emitted by the sun. 
 As changes in the quantity of heat received 
 can exercise directly so much more in- 
 fluence in the tropics than near the poles, 
 and consequently must be accompanied by 
 a considerable increase or diminution of 
 the general circulation of the atmosphere, 
 they will only be of small influence on the 
 
OF THE GEOLOGICAL PAST. 
 
 99 
 
 actual temperature of the air at the equator, 
 but, on the contrary, indirectly, of very 
 great influence upon the temperature at high 
 latitudes. 
 
 The postulated Changes of Solar Radition 
 have actually taken place. 
 
 As considerable changes in the solar 
 heat must consequently apparently suffice 
 to explain the observed climatic changes, 
 the question arises whether they have really 
 taken place. 
 
 This is actually the case and by these 
 changes we can satisfactorily explain the 
 known main phenomena of the geological 
 climates. 
 
 It has been repeatedly remarked, by 
 the way, that, as the sun is constantly 
 losing its energy, it must once have been 
 hotter than at present and may thus have 
 
IOO 
 
 THE CLIMATES 
 
 produced the higher temperature of the 
 early geological ages. * 
 
 As a general remark this, certainly, is 
 perfectly correct ; but a gradual cooling 
 down during the entire geological time has 
 not been observed in the climates of past 
 ages. During the long Jurassic and Cre- 
 taceous periods we see no marked change 
 with regard to the general conditions of 
 the heat over the earth, and it is now 
 accepted that the temperature in temperate 
 latitudes can have been but little higher 
 
 *F. Pfaff, Geologie als exacte Wissenschaft. Leipzig 1873, 
 pp. 38 and 39. 
 
 Sir William Thomson, Transactions Geol. Soc. of Glasgow. 
 Vol. 5, 1877, p. 238. 
 
 J. D. Dana. Manual of Geology. Third edition. New 
 York and Chicago. 1879, p. 716. 
 
 A. Geikie, Text-Book of Geology, Second Edition, London, 
 1885, pp. 19 and 21. 
 
 Compare also : J. J. Murphy, The Climates of Past Ages, 
 in: Nature, Vol. 42. 1890, p. 270, and: E. Bruckner, 
 Klimaschwankungen, p. 315. 
 
OF THE GEOLOGICAL PAST. 
 
 IOI 
 
 during the Palaeozoic age than during the 
 latter part of the Cretaceous period. On 
 the contrary the general coolings must 
 have progressed very rapidly during the 
 relatively short Tertiary period. 
 
 As far as I know no attempt has as yet 
 been made to compare the sun’s history 
 in full detail, as it must be according 
 to the now almost generally accepted and 
 everywhere so brilliantly confirmed theory 
 of Kant and Laplace, with the history of 
 the geological climates. Yet it appears to 
 me that, from the present state of science, 
 these last find a satisfactory explanation in 
 the history of the sun’s energy. 
 
 The sun is a star like the thousands 
 which appear at night as luminous specks 
 in the celestial vault, even of relatively 
 small dimensions and differing from the 
 others merely by its closer proximity to 
 the earth. The substances of which it 
 
102 
 
 THE CLIMATES 
 
 consists are entirely the same as those of 
 which many other stars are composed. 
 The spectrum of the yellow stars, belong- 
 ing to the second class of Vogel, in every 
 respect agrees with that of the sun, and 
 it may, therefore, be confidently affirmed 
 that the sun is a yellow star, and that the 
 other yellow stars not only have the same 
 composition, but also an almost equally 
 dense and hot atmosphere as our sun.* 
 From the colour and the spectrum of the 
 stars we not only learn their composition 
 but also their temperature, f for as soon as, 
 at a certain temperature, a body begins 
 to glow, the luminous and chemical rays 
 appear. The higher the temperature rises, 
 the richer the spectrum becomes in blue 
 and violet rays ; the lower the temperature 
 
 *A. Secchi, The Stars. 
 
 •j-J. Janssen, L’age des etoiles. Ciel et Terre. 2e S6rie. 3 e 
 annee. 1887 — 1888, p. 465 sqq. 
 
OF THE GEOLOGICAL PAST. 
 
 103 
 
 falls, the more the less refractible yellow 
 and red rays will predominate. A star 
 whose spectrum is very rich in violet rays 
 will, therefore, at least in its external 
 parts, possess a very high temperature. 
 The number of the stars belonging to this 
 class, of which Sirius, the most beautiful 
 of all, is the most brilliant, but Regulus 
 the most typical representative, is very 
 great. More than half — 58.5% — of all 
 known stars belong to the first class, 
 that of the white (bluish white) stars. 
 They possess a dense, hot atmosphere of 
 hydrogen, as seen by the four charac- 
 teristic broad lines in their spectrum; we 
 also observe in many stars of this class 
 a number of almost imperceptibly thin lines, 
 indicating the presence of metallic vapours, 
 chiefly of iron. 
 
 The stars of the second class, to which 
 our sun belongs, and with Capella as a 
 
104 
 
 THE CLIMATES 
 
 typical representative, possess no longer that, 
 bright white colour, but are yellowish 
 some even orange. From this we may 
 infer that they are less hot. Instead of 
 a mighty atmosphere of hydrogen, the 
 spectroscope shows us here, by very 
 numerous thin lines, the same metallic 
 vapours which we observe in our sun. 
 Hydrogen lines are still present, but they 
 are very thin and not at all so conspicuous 
 as in the white stars. This second class 
 is still very numerously represented, it 
 contains 33.5 °/o of all visible stars. 
 
 Other stars of a red hue, of which 
 Beteigeuze is a type, are in a still later 
 stage of evolution ; their spectrum indicates 
 unmistakably an already far advanced cooling. 
 The violet has entirely disappeared from 
 these broad dark bands or columns covering 
 the lines, similar to, but stronger and 
 more numerous than those of the second 
 
OF THE GEOLOGICAL PAST. IO5 
 
 class. To this class belong about 8°/o of 
 all the stars.* 
 
 * These figures, indicating the proportion in which the 
 stars are divided in the three spectral classes, are computed 
 from the Potsdam spectroscopic “ Durchmusterung ” (Public, 
 d. Astrophys. Observat. zu Potsdam. Vol. 3. Comp. : J. 
 Scheiner, Die Spectral-analyse der Gestirne. Leipzig 1890, 
 P- 325 ). 
 
 E. C. Pickering (Preparation and Discussion of the Draper 
 Catalogue. Annals of the Astronomical Observatory of Har- 
 vard College. Vol. 26. Part 1. 1891) has lately arrived at 
 a different conclusion, by way of photography. He found 
 for the figures indicating the proportion in which the three 
 types of star spectra are divided 75, 23 and 1 (p. 15 1). — 
 But the photographic plate is in this classification much more 
 one-sided than the eye, which is most sensitive to the 
 rays of the stars belonging to the middle class. Feeble stars 
 of the third class are, therefore, photographically never to be 
 distinguished from those of the second class (1. c., p. 178), 
 just as the classification is difficult for all bright stars 
 of less pure type, such as a Tauri and a Orionis, and 
 all the less bright stars (Op. cit. Vol. 27, Preface). The 
 Classification according to direct eye observations will there- 
 fore certainly express the real proportion more accurately. 
 
 Taking into account the one-sidedness of the photographic 
 plate (being chiefly sensitive to the rays of the violet side of 
 
io6 
 
 THE CLIMATES 
 
 Relatively few stars form the transition 
 from the first to the second class ; relatively 
 more numerous are those which, according 
 to their colour and spectrum, must be 
 ranged between the second and the third 
 class and therefore form the transition 
 from the second to the third stage. 
 
 Except the classes of stars mentioned, 
 which we must assume to have been 
 developed out of one another (and the 
 few stars which cannot be ranged in 
 these three classes), we further know, as the 
 
 the spectrum) by the arrangement of the spectral types, it 
 does not appear surprising that, as found by J. C. Kapteyn, 
 (See : Proceedings of the Royal Academy of Sciences at 
 Amsterdam on April 29, 1892 and his Memoir on Stellar 
 Magnitudes in Relation to the Milky Way, published in the 
 Bulletin du Comite International permanent pour l’execution 
 photographique de la carte du ciel, 1892) the number of the 
 stars of the first type — according to the Pickering photo- 
 graphic classification — in relation to the number of that 
 of the second type, increases in proportion to the distance 
 from us. 
 
OF THE GEOLOGICAL PAST. 
 
 107 
 
 first stage in the evolution of the luminous 
 celestial bodies, the nebulse, consisting of 
 very thin and voluminous gaseous masses, 
 which, according to the now current 
 hypotheses, originated by the falling of dark 
 bodies upon one another. In these nebulse 
 only hydrogen has as yet been proved to 
 exist with certainty. This class is so 
 limited in number that it contains less 
 thousands than the stars contain millions. 
 
 Besides all these visible bodies many 
 dark bodies (planets and extinguished 
 suns) must exist, — the number and the 
 total mass of which must even con- 
 siderably exceed those of the visible 
 bodies. 
 
 This universe has not existed for ever, 
 but must have had a beginning. If not, 
 the forces of nature would have attained 
 an equilibrium and we should not be able 
 to observe nature, as has been pointed 
 
io8 
 
 THE CLIMATES 
 
 out by Fick * and Secchi. f But the 
 universe which is known to us is not 
 infinite. If this were the case, says 
 .Secchi § with Olbers and Chesaux, we 
 ought to see the celestial vault in its 
 entirety as luminous as the sun, and 
 as this is not so, we are forced to the 
 conclusion that the number of stars cannot 
 be infinite. Secchi, however, acknowledges 
 that doubtless a great many dark bodies 
 exist in space which might intercept the 
 light, but by the great distances of the 
 luminous, as well as of the dark bodies, 
 the latter would, like the dust in our 
 atmosphere, be able to weaken the light, 
 but not to entirely absorb it. If these dark 
 
 *A. Fick, Die Naturkrafte in ihrer Wechselwirkung. 
 Wurzburg 1869, p. 70. 
 
 •j*A. Secchi in a lecture delivered March the 6th 1876 at 
 Rome on the Greatness of Creation with regard to space 
 and time. 
 
 §A. Secchi, The Stars. 
 
OF THE GEOLOGICAL PAST. 
 
 109 
 
 bodies are very numerous, as they really 
 must be, their intercepting and weakening 
 of the light may be so considerable that 
 the whole background of the heavenly 
 vault may be but very slightly lighted, and 
 form a dark screen against which the 
 stars, which we are nearest, will sparkle 
 like luminous specks. It does not neces- 
 sarily follow that, because there is an 
 apparent regularity in the construction of 
 this universe of stars, it is infinite. It is 
 quite possible for it to be of infinite extent 
 and yet fill only a limited part of space. 
 But as we are entirely unable to make obser- 
 vations through infinite space, we are by no 
 means entitled to infer a veritable creation, 
 an arising out of nothing, merely because of 
 the necessity we feel of assuming a commence- 
 ment to that universe known to us. 
 
 In any case it is highly probable that 
 our universe has, even in comparison with 
 
I IO THE CLIMATES 
 
 the duration of the longest star-life, existed 
 for an exceedingly long time, and that the 
 above sketched course of evolution of the 
 celestial bodies has been repeated a 
 countless number of times. As moreover 
 the number of stars is very large and their 
 masses must have been distributed by 
 chance — and as we may, further, assume 
 that the evolution of celestial bodies which 
 consist everywhere of the same substances 
 and are governed by the same forces takes 
 place always in a perfectly uniform manner 
 as has actually been proved by spectral 
 analysis (in all probability mainly according 
 to the nebular theory of Laplace), we are 
 able to estimate with tolerable accuracy 
 the duration of the typical stages from the 
 observed proportion which exists between 
 the number of stars of the different classes- 
 — the spectroscope enabling us to make 
 these observations, entirely independent of 
 
OF THE GEOLOGICAL PAST. 
 
 I I I 
 
 our subjective sense, of the colours and 
 of the relative luminous power of the stars. 
 Thus, with tolerable accuracy, the duration 
 of the white stage may be fixed at 58. 5 °/ 0 , 
 that of the yellow stage at 33.5°/ 0 and that 
 of the red stage at 8°/o of the mean total 
 luminous existence of a star. The nebular 
 stage must have lasted for so short a time 
 (less than o. i°/o of the total time of evo- 
 lution) that it need not be taken into 
 account, and the same with those stars which 
 exist in a state of transition between differ- 
 ent stages, their number being but 
 relatively small. 
 
 The great uniformity in the development 
 and in the composition of each class * 
 
 * A most remarkable proof of this extraordinary uniformity, 
 extending also to the conditions of density and temperature, 
 has been given by J. Scheiner in his “Die Spectralanalyse 
 der Gestime,” pp. 280 — 287. 
 
 As regards other conceptions relating to the evolution of 
 
I I 2 
 
 THE CLIMATES 
 
 justifies us in applying also to our sun the 
 mean proportion as allotted to the differ- 
 ent stages of star-life. The sun must have 
 passed the greater part of its existence in 
 the white stage, and, as it is at present 
 in its yellow stage, it must have passed 
 at least 58 . 5 °/o, or about 3 /b, of its star- 
 life. In which term of the yellow stage it 
 is now can neither astronomically nor phys- 
 ically be determined with any accuracy;* 
 but here we may look to palaeontology 
 for information. 
 
 the stars than that here set forth, which is founded on the 
 ground of the well confirmed nebular theory of Kant 
 and Laplace, such as were enunciated by J. Norman 
 Lockyer (The Meteoritic Hypothesis. London 1890) and also 
 by E. W. Maunder (Journ. Brit. Astron. Oct. 1891), I refer 
 to Scheiner (Op. cit., pp. 330 — 331). 
 
 *The spectralanalytic examination of the sun still proves 
 that the sun, like a Aurigae (Capella), which in all respects 
 resembles the sun and many other yellow stars, must be 
 a very young member of its class and is immediately connected 
 to the white stage by a few transitory stars like Procyon. 
 
OF THE GEOLOGICAL PAST. I I 3 
 
 During the longest period of the existence 
 of organisms on the earth, as far as our 
 knowledge extends, it must have received 
 considerably more heat than at present. 
 Hence the solar radiation must then have 
 been considerably more powerful, so much 
 more so than at present, that the sun must 
 necessarily have been in quite a different 
 condition. Now, since from the sun’s 
 history we learn that, during the greater 
 part of its existence as a white star, it 
 was much hotter, and on passing relatively 
 rapidly from the white to the yellow 
 stage, it lost much of its heat, we see, 
 on the other hand, that the heat received 
 by the earth underwent the same changes, 
 for after a very long period of warmth, 
 a relatively rapid cooling set in, finally 
 reaching the present condition — then we 
 may assume that the period of the cooling 
 down of the climates coincided with the 
 
 8 
 
THE CLIMATES 
 
 114 
 
 transition from the white to the yellow 
 stage, the period of rapid cooling of the 
 sun. The present thermal condition at 
 the earth’s surface was developed during 
 the Tertiary period and was fully attained 
 at the beginning of the Pleistocene period — 
 a date, geologically speaking, so recent, 
 that the time which has since elapsed 
 may perhaps be estimated as but one 
 fiftieth of the time which elapsed since 
 the beginning of the Palaeozoic age. We, 
 therefore, may further conclude that our 
 sun only comparatively recently entered 
 its yellow stage, and we may, without com- 
 mitting a great error, consider that it has 
 now passed 3 /5 of its life as a luminous 
 and heating star and that it has still 2 /s 
 before it. 
 
 According to the science of to-day the sun 
 draws nearly all its energy, as Helmholtz 
 first pointed out, from the gradual shrinking 
 
OF THE GEOLOGICAL PAST. 
 
 ”5 
 
 of its gaseous body, ever since the time of 
 the nebula, from which, according to the 
 theory of Kant and Laplace, it originated.* 
 Helmholtz and Lord Kelvin*)* calculated 
 the time during which the sun can have 
 been shining with its present intensity, 
 and how long it can still continue to 
 do so. Helmholtz estimated the past 
 period of solar heat at 20 million years. 
 His calculation was based on the older 
 value of the intensity of solar radiation 
 determined by Pouillet (Solar con- 
 
 *H. von Helmholtz, Vortrage und Reden. Vol. i. 
 Braunschweig 1884, p. 25: “Ueber die Wechselwirkung der 
 Naturkrafte,” lecture delivered in 1854. Ibid.: Yol. 2, p. 57: 
 “Ueber die Entstehung des Planetensy stems,” lecture delivered 
 in 1871. — The meteorites falling into the sun are to be 
 considered as quantitatively unessential for the maintenance 
 of solar energy. 
 
 ■J" Sir William Thomson, The Sun’s Heat. Proceed. Roy. 
 Institution of Great Britain, Vol. 12, 1887, p. 1 ; reprinted 
 in his : Popular Lectures and Addresses, Vol. 1 : Constitution 
 of Matter, second edition, London, 1891, pp. 376 — 429. 
 
THE CLIMATES 
 
 I 16 
 
 stan t= i. 7 6) ; according to later investiga- 
 tions by Langley (Solar constant=3)* the 
 sun could for 1 2 million years only have 
 emitted as much heat and light as at pre- 
 sent. Lord Kelvin thus came to the 
 conclusion that, taking into account every- 
 thing by which the computation could be 
 somewhat increased, “it would be exceed- 
 ingly rash to assume as probable anything 
 more than 20 million years of the sun’s 
 light in the past history of the earth or to 
 reckon on more than 5 or 6 million years 
 of sunlight for time to come. ” Hence the 
 
 *S. P. Langley, Researches on Solar Heat and its 
 Absorption by the Earth’s atmosphere. A Rapport of the 
 Mount Whitney Expedition. Professional Papers No. 15 of 
 the Signal Service. Washington 1884. — Crova (Comptes rendus 
 de l’Academie des Sciences. Paris. T. 108. 1889, p. 35) 
 Sawelief (ibid, p. 287; T. no, p. 235, T. 112, pp. 481 
 and 1200) and R. Angstrom (Wochenschrift fur Astronomie 
 und Meteorologie. 1890, Nos. 14 — 16) concluded still higher 
 values for the Solar Constant than Langley. 
 
OF THE GEOLOGICAL PAST. I I J 
 
 sun is at present in such a state of con- 
 traction that it has spent already from 
 3|- to 4 times as much heat as it has 
 left. 
 
 The sun’s yearly loss of heat must, 
 however, have been much larger during its 
 white stage than in its present condition. 
 It can, therefore, only have been shining 
 for a much shorter period than 20 mil- 
 lion years. Distributing a quantity of 
 heat from 3-i a to 4 a over 58.5 time 
 parts of the white stage, and the quantity 
 of heat 1 ci of the collective yellow and 
 red stages over 41.5 time parts, we find 
 that during the white stage the sun must 
 have developed on an average a 2.36 to 
 2.84-fold intensity of radiation as that 
 which it has available for the yellow and 
 red stages. We thus further find for the 
 white stage of our sun 7 to 8^ million 
 years, — assuming that the sun will keep 
 
I I 8 THE CLIMATES 
 
 shining with its present energy up to the 
 end of its luminous existence. In reality, 
 however, the solar radiation will be consid- 
 erably less intense during its red stage, 
 that is, during one fifth part of its future 
 existence as a luminous star. Assuming 
 for the intensity of radiation in the red 
 stage the smallest theoretically possible 
 value, namely zero, then we obtain for 
 the white stage, as a minimum value, 
 from 1.9 1 to 2.29 times its amount in 
 the yellow stage, which means for the 
 white stage a period of 8f to 10J million 
 years. But in reality the intensity of 
 radiation of the red stars is by no means 
 equal to zero.* 
 
 *Judging by their spectrum the sun-spots must be in a 
 condition very like that of the red stars (Scheiner, Die 
 Spectralanalyse der Gestirne, p. 315). Now Langley has 
 found that they emit about 54°/* of the heat emitted by an 
 equally large part of the photosphere (C. A. Young, The 
 Sun). As, however, the rays emanating from the red stars 
 
OF THE GEOLOGICAL PAST. 
 
 HQ 
 
 We have, further, every reason to suppose 
 that, just as in the white stage (judging 
 from the existence of a numerously repre- 
 sented white class as well as from the 
 general stability of the Pretertiary climates), 
 so also during the yellow stage the radiation 
 of the sun will diminish but very slowly. 
 When Lord Kelvin calculated the future 
 amount of sun heat at 5 or 6 million 
 years, he had, moreover, already taken 
 into account the possibility of a reduced 
 radiation. Hence the past period of solar 
 heat cannot have lasted much longer, and 
 
 and from the sun spots undergo slightly less absorption in 
 our atmosphere than those emanating from the yellow stars 
 and the photosphere of our sun, we may conclude that the 
 red stars possess about half the intensity of radiation of the 
 yellow stars. By this assumption we find a radiation in the 
 white stage, from two to two and a half times that in the 
 yellow stage, and we further find for the first stage of our sun 
 a period of 8 to io million years, for the yellow stage of 
 4 */a to 57a million and for the red stage of i to 1 7a mil- 
 lion years. 
 
I 20 
 
 THE CLIMATES 
 
 we cannot be far wrong in assuming for 
 the past a maximum duration of ten 
 million years, and a radiation in the 
 white stage twice as intense as the present. * 
 This excess of solar heat must certainly 
 
 * Repeated attempts have been made to arrive at some 
 conclusion about the relative intensity of radiation of stars belong- 
 ing to the first and second spectral classes, from the photometric 
 data concerning stars of which the distance and the mass 
 could be approximately calculated and which could thus be 
 compared to our sun. The unreliability of these data is, 
 however, so great that it is quite impossible to arrive at 
 any trustworthy results from them, even for stars like cc 
 Centauri, for which the data differ the least. (Compare the 
 Tables of fixed-star parallaxes by J. A. C. Oudemans in: 
 Astron. Nachr. 1890, Nos. 2915 — 2916; more unreliable 
 still are the computations about the mass of the double 
 stars). Hence the great differences in results. Recently E. 
 W . Maunder (Joum. Brit. Astron. Soc. Oct. 1891) found, 
 in comparing the luminous power of nine stars of the first 
 class and thirteen stars of the second class, whose parallaxes 
 were determined by heliometer measurements by Elkin, that 
 the luminous power of the stars of the first class is in the 
 mean only equal to 2 /s of that of the second class. But 
 probably the white stars have still greater relative luminous 
 
OF THE GEOLOGICAL PAST. 
 
 I 2 I 
 
 power, and to this circumstance it must certainly be ascribed 
 that the total luminous power of the white stars of first and 
 second magnitude exceeds somewhat more that of the yellow 
 stars of the same magnitudes than is the case with the 
 stars of smaller magnitude, taking into account the arithmetical 
 proportion of the two spectral classes. 
 
 Then, again, photometrical comparisons do not give much 
 information about the relative intensities of radiation; for 
 not only are the rays emitted by the bluish white stars, 
 absorbed in our atmosphere in quite a different manner from 
 those of the yellow stars, but also our eye is not as sensitive 
 to blue as to yellow rays, and is almost entirely blind to 
 a great part of the rays emitted by bluish stars. More 
 sensitive to, and thus a better measure for the real radiation 
 of the white stars is the photographic plate; the maximum 
 of the intensity of radiation of the spectrum of these stars 
 must nearly coincide with the photographic maximum in the 
 spectrum. It appears, however, that the difference between 
 the photographic and the optical brightness of the white 
 stars is but very small — smaller than o.i magnitude according 
 to Kapteyn (Proceedings of the R. Acad, of Sciences at 
 Amsterdam from April 2, 1892) — which is to be accounted 
 for by the fact that more than half of the photographic 
 rays are absorbed in our atmosphere. Of the rays of the 
 yellow stars, however, but one fourth is absorbed. Moreover 
 the eye is at least twice as sensitive to yellow as to 
 blue rays. 
 
 Huggins and Stone tried (about 20 years ago) to measure 
 
122 
 
 THE CLIMATES 
 
 have been sufficient to exercise a noticeable 
 effect on the general thermal conditions of 
 the earth. 
 
 directly the radiating heat of the stars. These first attempts 
 made with the aid of the thermopile and the galvanometer 
 were, however, lately repeated with a much more sensitive 
 instrument of a different construction, the radiomicrometer, by C. 
 V. Boys (Proceedings of the Royal Society. Vol. 47. London 
 1891, p. 480), with the result that, notwithstanding the 
 extraordinarily high sensitiveness of the instrument, no trace 
 of an existing star-radiation could be detected. 
 
 It, however, can be deduced, with perfect certainty from 
 all our present data, that the specific radiating capacity of 
 stars of different spectral classes cannot differ considerably, 
 and that the white stars can be, and very probably are, at 
 most, only a few times hotter than the yellow stars and 
 these again than the red stars. This surprising conformity 
 with the radiated heat arrived at in the present essay in 
 quite a different manner (the computation might indeed have 
 given for result 10, 1 00 or 1000 times higher values), proves 
 as well the correctness of the Helmholtz contraction theory 
 of the sun’s heat, as also of the assumption that the sun 
 only recently, geologically speaking, viz. at the end of the 
 Tertiary period, became a yellow star, and this seems to me 
 to be a strong argument that the explanation of the geological 
 climatic changes given here is based on a sure foundation. 
 
OF THE GEOLOGICAL PAST. 
 
 123 
 
 Now, the atmosphere possesses, in its 
 variable amount of aqueous vapour and in 
 its changeable cloudiness, powerful means 
 of regulating the quantity of heat arriving 
 at the surface of the earth, and also its 
 diffusion over the earth in time and space. 
 At the time when the sun still belonged 
 to the stars of the first spectral class,* 
 the earth must have probably been in a 
 similar condition to that in which, ac- 
 cording to the astronomers and the spec- 
 tralanalysts, our sister-planet Venus now is, 
 which receiving about twice as much ra- 
 diated heat as the earth, has a much 
 denser atmosphere, very rich in aqueous 
 vapour and in its higher parts filled with 
 a thick layer of clouds. The thick layer 
 
 ♦Only the albedo of the earth’s atmosphere must, because 
 of the greater coefficient of absorption of the sun’s rays then, 
 have been much smaller than the present albedo of Venus, 
 which is determined by Zollner and Seidel at 0.6. 
 
124 
 
 THE CLIMATES 
 
 of clouds must then have reflected a 
 great part of the sun’s rays and at the 
 same time this must, together with the 
 aqueous vapour contained in the atmos- 
 phere, have made the temperatures more 
 equal in time and space. 
 
 That part of the surplus of radiated 
 heat, absorbed by the atmosphere, will 
 almost entirely have benefited the higher 
 latitudes, owing to the stronger general 
 circulation of the air which it must have 
 caused. Moreover, it is of great signifi- 
 cance that the sun in its white stage pos- 
 sessed not only a much intenser radiation, 
 but also in the form of rays of a reduced 
 wave length. For these, according to the 
 investigations of Langley, are absorbed 
 in a much higher degree by the earth’s 
 atmosphere than the rays of greater wave 
 length. Thus from the white sun, so 
 much richer in violet and ultra-violet 
 
OF THE GEOLOGICAL PAST. 
 
 125 
 
 rays, far fewer rays will have reached 
 the earth’s surface than from the present 
 sun, being in the condition of a yellow 
 star and possessing relatively many 
 red and ultra-red rays. Hence a rela- 
 tively much smaller part of the energy 
 of the white sun could by direct radi- 
 ation influence the temperature of the 
 climate anywhere, but especially near the 
 equator. On the contrary, the energy of 
 the blue, violet and ultra-violet rays, which 
 were then preponderating, must, by con- 
 tinuous absorption, have been transferred 
 to the higher atmospheric strata and used 
 in increasing the general circulation of the 
 atmosphere and thus, in an indirect way, 
 have benefited the subpolar latitudes. 
 
 If, on the contrary, the sun were to be 
 in the less warm condition of a red star, 
 a relatively small part of its energy of 
 radiation would be spent in maintaining 
 
THE CLIMATES 
 
 I 2 6 
 
 the general air circulation. The higher 
 latitudes would then have been indirectly 
 much less warmed than now, although, 
 near the equator also, the temperature 
 would be lower, the increase of red and 
 ultra-red rays being smaller than the 
 diminution of the violet and ultra-violet 
 rays. 
 
 The comparatively small number of stars 
 which, like Procyon, form the transition 
 from the first to the second stage, must 
 correspond to the relatively rapid gradual 
 cooling of the climates of the Tertiary 
 period. But at the beginning of the Pleis- 
 tocene period, when the reign of the 
 yellow sun had commenced and the present 
 climatic conditions had already set in, twice 
 or thrice a considerable temporary cooling 
 took place, leading to glaciation everywhere, 
 where favourable conditions existed. These 
 Pleistocene coolings can not have been 
 
OF THE GEOLOGICAL PAST. 
 
 127 
 
 much less than the rise of the tempera- 
 ture was during the Pretertiary ages, 
 and must correspond to very considerable 
 diminutions of the solar radiation, which 
 again, being accompanied by a great 
 disturbance in the conditions of the sun, 
 would have become apparent by its colour 
 and its spectrum. 
 
 What may then be the stars, at present 
 in this glacial intermediate period of the 
 second stage? Apparently no other than 
 transition-stars from the second to the 
 third class, or stars belonging to the third 
 class itself. * The actual occurrence of 
 
 * More probably transition-stars, — firstly, because the 
 difference between the temperature of the present climate 
 at temperate latitudes, and that obtaining during the Glacial 
 epoch, is smaller than that of the middle Tertiary 
 (the former being only about the half of the latter), while 
 the red stars are nearly as much cooler than the yellow 
 stars, as the white stars are warmer than the latter; secondly, 
 because during the Pleistocene Glacial epoch the difference 
 
128 
 
 THE CLIMATES 
 
 such considerable periodical changes in 
 the sun’s radiation is rendered probable 
 by the relatively great number of tran- 
 sition-stars between the second and the 
 third class, by the fact that a number 
 of yellow stars undergo periodical changes 
 of shorter duration, by which the breadth 
 and the obscurity of the lines increase and 
 even traces of column-shaped zones appear, 
 like those characteristic of stars belonging 
 to the third class. Further, by the fact 
 
 of temperature with the present was only about four times 
 as great as the oscillation of the temperature during the 
 35 yearly period discovered by Bruckner, of which no 
 corresponding changes in the sun have as yet been discovered, 
 though there are many indications that this is also caused 
 by variations in the solar radiation ; thirdly, because the earth 
 is at present in a period somewhat cooler than the middle 
 Interglacial periods and corresponding climatically more with 
 the transition periods between the Glacial and Interglacial 
 periods, (Bruckner, Klimaschwankungen, p. 3 1 4), while the 
 sun represents at present the pure type of a yellow star. 
 
OF THE GEOLOGICAL PAST. I 29 
 
 that many variable stars belong to the 
 transition group and most of them to 
 the third class, and lastly in the body 
 of our sun short periodical changes 
 take place, of which at least an eleven- 
 year period of sun-spots is proved to a 
 certainty. When such periodical changes 
 of short duration take place, periods of 
 longer duration become probable, with 
 which not only the statistical data regarding 
 the different classes of stars are in accord, 
 but also our knowledge of the physical 
 and chemical conditions of the sun, as 
 well as the geological phenomena, give 
 indications. Since the sun entered its present, 
 already considerably cooler state of a 
 yellow star, thereby causing the appearance 
 in its atmosphere of a great number of 
 chemical elements, conditions favourable 
 for periodical changes in its radiation 
 were certainly created by the possibility 
 
 9 
 
130 
 
 THE CLIMATES 
 
 of chemical combinations being formed. 
 In the last years it has been repeatedly 
 pointed out that the variability of the 
 eleven-year period of sun-spots may also 
 be chemically explained. The geological 
 phenomena of the Pleistocene period can 
 in this way be well explained. Though 
 the estimates for the duration of the whole 
 period have led to very different results, 
 it may be taken for granted that the 
 Interglacial period must have been of very 
 long duration, every one much longer than 
 the Postglacial period in which we are 
 now living. This may, therefore, quite 
 well, even as far as regards its duration, 
 be looked upon as the equivalent of the 
 Interglacial periods with which it entirely 
 agrees in its climatic conditions. It is 
 likewise probable that the Interglacial 
 periods have continued absolutely longer 
 than the Glacial epochs, and that thus 
 
OF THE GEOLOGICAL PAST. 131 
 
 their total duration — past and future — may 
 be compared to the number of yellow 
 stars, and that of the collective Glacial 
 epochs to the number of the transition 
 stars to the red (perhaps, but improbably, 
 the red stars themselves included). Every- 
 thing seems to indicate that during the 
 yellow stage in long oscillations, always 
 during a relatively short time, chemical 
 combinations occur by which the colour 
 of the star becomes reddish (or red), and 
 broader and darker lines, indicative of a 
 denser atmosphere, and dark bands or 
 columns, indicative of chemical combinations, 
 appear in the spectrum. By the otherwise 
 unimportant and regular decrease of the 
 radiation during the yellow stage, these 
 oscillations will repeat themselves probably 
 for a long time without any considerable 
 lengthening or shortening of the inter- 
 mediate periods, and only shortly before 
 
132 
 
 THE CLIMATES 
 
 the end of the sun’s life the intermittent 
 cool period will grow rapidly and the 
 body of the sun grow permanently red 
 and at last dark. 
 
 The Solar Heat arises most probably 
 from Contraction. Seeming Incon- 
 sistency with Geological and 
 Biological Data. 
 
 According to Helmholtz and Lord 
 Kelvin we must look for the origin of 
 solar heat almost exclusively to the poten- 
 tial energy which the sun possesses by 
 the mutual attraction of its component 
 parts. Before the matter of which the sun 
 consists coalesced and became hot, it must 
 have existed in space in the form of cold 
 masses. According to Helmholtz and 
 Lord Kelvin the force which caused them 
 to coalesce lay in the gravitation or gen- 
 eral mutual attraction of the celestial bodies, 
 
OF THE GEOLOGICAL PAST. 1 33 
 
 and on this supposition they based their 
 calculations of the duration of the sun’s 
 heat. Lord Kelvin * enquires whether other 
 forces might not have caused the coalescence 
 of the sun’s mass. There might have been, 
 so says the great physicist, two cold bodies 
 which came into contact by the velocity 
 caused by their mutual attraction. “With 
 enormously less of probability” there might 
 have been two masses which collided with 
 considerably greater velocities than the 
 velocities due to mutual gravitation. In 
 this case their own motions ought to have 
 been almost perfectly directed towards each 
 other’s centre of inertia, for, even with 
 relatively small velocities, a slight deviation 
 from the line joining the centre would 
 prevent the collision or cause the already 
 collided bodies to separate again and make 
 
 Sir William Thomson, The Sun’s Heat, l.c. p. 13 sqq. 
 
134 
 
 THE CLIMATES 
 
 them revolve round the common centre of 
 inertia. 
 
 “ The dry probability of collision between 
 two neighbours of a vast number of mu- 
 tually attracting bodies widely scattered 
 through space is much greater if the 
 bodies be all given at rest, than if they 
 be given moving in any random directions 
 and with any velocities considerable in 
 comparison with the velocities which they 
 would acquire in falling from rest into 
 collision.” In the latter case the probabil- 
 ity of collision will be smaller the greater 
 the velocities of their own motions are. 
 In reality many stars and also our sun 
 have a motion of their own, whose veloc- 
 ity is, however, generally very small 
 compared to that which a body would 
 acquire by the attraction of the sun. While 
 the latter is 612 K.M. per second, there 
 appear in the firmament only few motions 
 
OF THE GEOLOGICAL PAST. 
 
 *35 
 
 of greater velocities than the tenth part 
 of this, and that of most stars is much 
 smaller. * Only a very few stars, as 
 1830 Groombridge, 9352 Lacaille, 31 
 Cygni and a few others have so rapid a 
 motion that it seems this cannot have 
 been caused by gravitation alone, at least, 
 taking, with Newcomb, f the visible stars 
 only into account. But in reality the total 
 mass and number of the dark bodies in 
 space is probably much greater than 
 that of the luminous bodies : the stars, as 
 
 "‘According to photographic spectral observations by H. C. 
 Vogel at Potsdam, and also accurate direct measurements 
 of displacements of spectra by J. Keeler of the Lick- 
 observatory at Mount-Hamilton (Publications of the Astronom- 
 ical Society of the Pacific, II, p. 265), it is apparent that 
 the velocity of the motions of a great number of stars is 
 much smaller than had been determined formerly, for instance 
 at Greenwich. On an average it was fpund to be only the 
 half of that, namely 1 7 K. M. according to the determinations 
 at Potsdam. 
 
 f Newcomb, Popular Astronomy, 1878, p. 487. 
 
THE CLIMATES 
 
 136 
 
 these can only be formed by a rare coin- 
 cidence of favourable conditions. It may 
 therefore be that even 1830 Groombridge 
 owes its velocity to gravitation alone. 
 However, relatively considerable velocities 
 are of very rare occurrence in the firma- 
 ment, and therefore it becomes still more 
 improbable that these could in an impor- 
 tant measure account for the energy of 
 the sun and of the other stars. 
 
 Lord Kelvin supposes further, that our 
 sun may have originated from a great 
 many bodies moving in different directions 
 and even then the probability of collision 
 must become smaller the greater the resul- 
 tant moment is of these velocities. In 
 that case also the most favourable condi- 
 tion for collision is that of rest and the 
 most unfavourable that of great velocities 
 of proper motion. 
 
 In opposition to this gravitation hypoth- 
 
OF THE GEOLOGICAL PAST. 
 
 137 
 
 esis, which is based on the theory of 
 probabilities, J. Croll poses his impact 
 hypothesis.* He believes that the bodies 
 of which the sun is composed had a motion 
 of their own, much greater than that which 
 could possibly have been caused by gravit- 
 ation alone, and — though acknowledging that 
 a collision would then be much more im- 
 probable — he perceives in this very improb- 
 ability the proof of the correctness of his 
 assumption. According to his opinion col- 
 lisions must be very rare, or else the uni- 
 verse could not exist in the condition which 
 we actually observe. It appears to me 
 that the grounds on which he bases his 
 assumption are extremely weak, for the 
 
 *See: Climate and Time in their Geological Relations. 
 London 1875, p. 353 sqq. ; Discussions on Climate and 
 Cosmology. Edinburgh 1886, Chapters XVIII and XIX and: 
 Stellar Evolution and its Relations to Geological Time. 
 London 1889. 
 
138 
 
 THE CLIMATES 
 
 probability of a collision of celestial bodies 
 having the velocities he supposes becomes 
 so exceedingly small, that, notwithstanding 
 the great number of dark bodies in the 
 universe and the long duration of individual 
 star-life, scarcely any star could be seen. 
 Croll has been led to this “ impact-theory ” 
 by his hypothesis for explaining the Pleis- 
 tocene glaciation. This hypothesis requires 
 a longer time for the existence of living 
 beings upon the earth than the solar 
 heat can possibly last, supposing this 
 to have originated from the generally 
 accepted source. The chief proof for 
 the correctness of his views Croll finds 
 in this, that geologists and biologists 
 require a longer time for the formation of 
 the geological strata and for the develop- 
 ment of life upon the earth than the 20 
 million years which the physicists could 
 grant as maximum. By measuring the 
 
OF THE GEOLOGICAL PAST. 
 
 139 
 
 sediments deposited by the rivers we can 
 estimate the average denudation of the 
 continents, and thus, supposing the present 
 conditions to have prevailed in the past 
 also, we can compute the time required 
 for the deposition of those sedimentary 
 strata of which the mountains of the 
 earth are now composed. These esti- 
 mates are, however, but very approxi- 
 mate and must lead to very different re- 
 sults. It is, further, possible and even very 
 probable that in former ages denudation 
 took place to a far greater extent than 
 now. It is dependent in the first place 
 on the quantity of rain and conse- 
 quently on the dampness of the air. De- 
 nudation and the disintegration of the 
 rocks, which accompanies it, are undoubt- 
 edly most considerable in the tropics, and 
 we actually notice that the rainfall greatly 
 increases towards the equator; according 
 
140 
 
 THE CLIMATES 
 
 to Murray, it is from two to three times 
 as heavy in the tropics as in temperate 
 latitudes. * At the time of the white sun 
 radiating so much more heat, the amount 
 of vapour contained in the atmosphere 
 must have been exceedingly great, and over 
 the whole earth, for when the regions in 
 high latitudes were, compared with the 
 tropics, still relatively short of heat directly 
 received, this was compensated by very 
 warm ocean-currents and mild winds, the 
 vapour of which must have condensed 
 abundantly on the land. The weathering 
 processes are, moreover (the moistening 
 relatively unaltered), really promoted by a 
 
 *J. Murray, On the total annual Rainfall on the Land 
 of the Globe. Scottish Geographical Magazine. Vol. 3. 
 1887, p. 65. 
 
 In the Malay Archipelago the rainfall is, for an equal area, 
 even five times, at the equator, at an average, four times as 
 large as it is in Europe. Moreover, the rainwater in the 
 tropics contains ten times as much nitric acid as in Europe. 
 
OF THE GEOLOGICAL PAST. 141 
 
 higher temperature, and, besides, the 
 vegetation is an important factor.* Dur- 
 ing the time when the sun still poured forth 
 an intenser flood of heat on the earth, 
 the circulation of the water must conse- 
 quently have been more powerful, and 
 therefore its mechanical effects of erosion 
 and of transport of the dissolved and solid 
 products of weathering more considerable. 
 We shall, therefore, certainly not be exag- 
 gerating in assuming that at that time 
 denudation may have been ten times as 
 strong as it is now in temperate lati- 
 
 *F. von Richthofen in: G. Neumayer’s Anleitung zu 
 wissenschaftlichen Beobachtungen auf Reisen. 2. Auflage. 
 Vol. I Berlin 1888, pp. 230 — 231. — Compare also: J. C. 
 Russell, Subaerial Decay of Rocks. Bulletin of the U. S. 
 Geol. Survey. No. 52. Washington, 1889. 
 
 After Muntz has shown that Bacteria (Nitromonas) are 
 the chief agents in the weathering of the rocks, this influence 
 of the moisture and the temperature has become quite in- 
 telligible. 
 
142 
 
 THE CLIMATES 
 
 tudes — and hence the computations of 
 geological time are not so reliable as Croll 
 and others suppose them to be. 
 
 On biological grounds we may rela- 
 tively more readily accept a very long 
 duration of the solar heat. The evo- 
 lution of the organic world progresses 
 extremely slowly. The development of 
 the three-toed Hyracotherium ( Orohippus) 
 into the true horse, which can be well 
 traced, required almost the whole of the 
 Tertiary period. Bats and whales, these 
 strange modifications of the mammal type, 
 are already found completely developed in 
 the Eocene formation, and at this rate the 
 long time from the beginning of the Pa- 
 laeozoic epoch would certainly not have 
 been sufficient to account for the develop- 
 ment of the whole class of mammals. 
 T he Multituberculata , geologically the oldest 
 known mammals, already existed in the 
 
OF THE GEOLOGICAL PAST. 
 
 143 
 
 Triassic period, although probably related 
 and perhaps belonging to the low- 
 est organised branch : the Monotremata, 
 which still stand morphologically so far 
 above the lower organised classes of 
 vertebrata and are from their first 
 appearance so much specialised, that, 
 with Wallace, we must date the origin 
 of the mammals, at the lowest estimate, 
 far back into the Palaeozoic age. * Accor- 
 ding to Marsh and Furbringer the same 
 is very probably the case with the birds, f 
 The commencement of power of flight 
 in the ancestors of this wonderfully dif- 
 
 * A. R. Wallace, Island Life. London 1880, p. 205. — 
 Compare also the beautiful address on Palaeontology and 
 the Doctrine of Evolution by Th. H. Huxley. Anniversary 
 Address to the Geological Society of London in 1870. 
 Proceed. Geol. Society, 1870. 
 
 f O. C. Marsh, Amer. Journ. of Science, (3) Vol. 22, 
 188 r. — Max Furbringer, Untersuchungen zur Morphologie 
 und Systematik der Vogel. Amsterdam, 1888, p. 1571. 
 
144 
 
 THE CLIMATES 
 
 ferentiated class certainly occurred in 
 the beginning of the Jurassic, perhaps in 
 the Triassic, period. * We have as yet 
 been able to follow the evolution and 
 detect the origin of smaller groups only 
 and not of the chief types or classes. The 
 more we get acquainted with early life, 
 the farther back in the past we must place 
 the first appearance of the higher forms. 
 As to plants, the Dicotyledons were first 
 supposed to have originated in the Ter- 
 tiary period, afterwards in the Cretaceous 
 period, and now this high class has been 
 found in the Jurassic formation also.f 
 As a rule the oldest representatives of 
 any class do not represent the beginning 
 of its evolution, and Darwin considers it 
 indisputable, that before the formation of 
 
 * Furbringer, l.c. 
 
 f G. DE Saporta in : Revue generate de Botanique, Vol. 
 2, 1890. 
 
OF THE GEOLOGICAL PAST. 
 
 145 
 
 the lowest Cambrian strata long periods 
 elapsed, as long as, or probably for longer 
 than, the whole time which has elapsed since 
 the Cambrian period until now, and that 
 during those immeasurable periods the 
 earth swarmed with living creatures. 
 Wallace, who perhaps more than any 
 other is entitled to express an opinion in 
 this matter, thinks that the time which has 
 elapsed since the first appearance of life on 
 earth cannot have been much less than twice 
 and a half the time which has elapsed since 
 the Cambrian period, and that, in all pro- 
 bability, it has been even longer, for, as is 
 believed, the reaction of the organisms on 
 changes in the surrounding conditions must 
 have been less intense in low and simple 
 than in high and complex forms of life. 
 Hence the organic evolution may for countless 
 ages have progressed extraordinarily slowly.* 
 
 ♦Wallace, Island Life, 1880, p. 205. 
 
 IO 
 
146 
 
 THE CLIMATES 
 
 It may, nevertheless, have been that 
 organic evolution under the reign of the 
 white sun progressed faster than for in- 
 stance in the Tertiary period, in which we 
 can accurately follow the evolution of some 
 forms, and when the light and heat of the 
 sun were already diminishing. The evolution 
 depends in the last instance on the appearance 
 of numerous and manysided hereditary in- 
 dividual modifications of the type, and the 
 mean form of every species and what was 
 probably the principal favourable condition 
 for these physiological and morphological 
 modifications lies, according to Darwin, 
 in an excess of nutrition;* yet just during 
 the Tertiary period the conditions of nutri- 
 tion underwent a continual limitation. The 
 more favourable Pretertiary conditions may 
 have acted in yet another way to promote 
 
 * Chapter XXII in : The Variation of Animals and Plants 
 under Domestication. 
 
OF THE GEOLOGICAL PAST. 
 
 147 
 
 organic evolution, namely by the increase 
 of the number of individuals of every species 
 available for the selection, while with the 
 number of the individuals the variability 
 itself must have increased in a high degree, 
 according to Weismann's heredity theory, 
 in consequence of increased intercrossing. 
 The damp uniform warmth of the atmo- 
 sphere and the greater intensity of the 
 sunlight must have everywhere to a great 
 extent promoted vegetation, and thereby 
 animal life. Palaeontology affords many 
 direct evidences that a richer development 
 of the latter has really existed. At least 
 from the middle of the Tertiary period 
 the organic evolution must, therefore, have 
 progressed considerably slower.* 
 
 * Principally towards the end of the Tertiary period, when 
 the sun passed rapidly into its yellow and then into its 
 Glacial stage. Here then is a plausible solution of the great 
 enigma, as Neumayr calls the phenomenon, of the extinction 
 
148 
 
 THE CLIMATES 
 
 over the whole earth of the rich Tertiary mammal-fauna and 
 its numerous great Diluvial descendants, owing to which 
 we now live in the midst of a very impoverished fauna — 
 “an occurrence which, however, shows us that in some not 
 yet recognisable manner and by still unknown causes, important 
 changes in the conditions of life prevailed over the great 
 majority of continents of the earth during the Diluvial 
 period, influences of more general signification and extent 
 than any causes ” (such as too far going specialisation, direct 
 influence of the Ice age or extermination by man) “which 
 one would be inclined to assume for them (M. Neumayr, 
 Die Stamme des Thierreiches. Wien und Prag. 1889, p. 
 1 49). By diminished variability and limitation of the material 
 for the selection the continually necessary faculty of adap- 
 tation — that without noticeable morphological variations in- 
 cluded — to changed external conditions must have decreased 
 more and more, yet at that time these external conditions 
 underwent such important changes in consequence of the 
 diminished solar radiation. In the long run this must neces- 
 sarily have led to the extinction of many species, in the first 
 place of those of very large animals, such being poorer in 
 individuals, less reproductive and requiring a greater quantity 
 of food. Only in this way does it appear conceivable how 
 so many species in all parts of the world became compara- 
 tively rapidly totally extinct without a struggle with the 
 victoriously competing organisms, and that not merely a 
 diminution in the number of individuals of every species has 
 taken place. 
 
OF THE GEOLOGICAL PAST. 1 49 
 
 Animal life also may have been directly 
 promoted by the intense light of the white 
 sun. Moleschott, Pfluger, von Platen 
 and others had formerly found that light 
 promotes the vital combustion in animal 
 organisms and tissues. Moleschott and 
 Fubini had, further, observed that even 
 with blind animals blue-violet light is 
 quite as active as white light, while red 
 light has much less effect.* Still later 
 Fubini and Spalitta| observed that the 
 minimum effect of blue and violet light 
 was in mammals and birds, though in toads 
 these colours had the maximum effect. 
 Pfluger had suggested before that this 
 strong increase of vital combustion might be 
 caused indirectly by reflectorily excited 
 muscular motions or by the intensifying 
 of the muscular tonus. The experiments 
 
 * Moles chott’s Untersuchungen. Vol. 12, pp. 266 — 428. 
 | Ibid. Vol. 13, p. 563. 
 
THE CLIMATES 
 
 150 
 
 of Loeb with the pupa of butterflies* have 
 really proved that the oxidation in the 
 tissues increases only slightly when muscular 
 motions and changes of the tonus are 
 excluded. The intensified vital combustion 
 must still to a small degree be ascribed 
 to the direct influence of light, and Mole- 
 schott and Fubini — as already mentioned — 
 observed that even blind animals were 
 affected by light, though in a less degree; 
 which was later also confirmed by Martin 
 and FRiEDENWALD.f It may, therefore, 
 be taken for granted that light generally also 
 directly promotes vital combustion, and 
 it is almost certain that this action is 
 mainly due to the more refrangible rays 
 
 ♦PflAger’s Archiv. Vol. 42, p. 393. 
 fH. N. Martin and J. Friedenwald in: Studies from 
 the Biological Laboratory of the John Hopkin’s University, 
 Baltimore, Vol. 4. 1890. 
 
OF THE GEOLOGICAL PAST. I 5 I 
 
 at the violet end of the spectrum, the so- 
 called chemical rays.* 
 
 Moreover, the influence of light on the 
 growth of plants is well known. This 
 appears mostly in the hindrance of longi- 
 tudinal growth (etiolation and positive 
 heliotropism), less often in promoting 
 it (negative heliotropism). This influence 
 is nearly entirely exercised by the rays of 
 greater refrangibility, and its maximum 
 corresponds to the transition from the violet 
 to the ultra-violet. So here again we 
 find a pronounced influence on the 
 vital interchange of matter of light in 
 
 * E. Yung (Archive des sciences physiques et naturelles. 
 Geneve, 1 883, pp. 5 5 — 56) observed the development of eggs of 
 fishes and frogs to be mostly promoted by blue and violet 
 light. — For the heliotropism of pelagic animals the stronger 
 refrangible rays of the spectrum proved to be more active 
 than the less refrangible rays. (T. J. Groom and J. Loeb 
 in: Biologisches Centralblatt. Vol. 10. p. 1890). 
 
152 
 
 THE CLIMATES 
 
 general, and principally of the “ chemical ” 
 rays of the spectrum. 
 
 The vital interchange of matter and 
 consequently all the vital processes — one 
 of which is the phylogenetic evolution — may 
 therefore have been more marked in animals 
 and plants under the white sun. 
 
 In inorganic nature the substances which 
 are sensitive to light are decomposed 
 almost exclusively by the more refrangible 
 rays of the spectrum, chiefly the blue, the 
 indigo, the violet and ultra-violet rays. 
 Now it is very remarkable that, on the 
 contrary, the decomposition processes of the 
 principal substances, or by means of these 
 substances which are sensitive to light in 
 organic nature — the “Sehpurpur” in the 
 retina of the eye and the chlorophyll in 
 the green parts of plants which establish 
 such an intimate relation between light and 
 life, decidedly the most intimate between 
 
OF THE GEOLOGICAL PAST. 
 
 153 
 
 any phenomenon of the surrounding world 
 and its organism, are not chiefly affected 
 by the “ chemical ” rays at the violet end 
 of the sun-spectrum, but by the most 
 energetic rays. The curve representing the 
 value of brightness of the colours of the 
 spectrum attains its zenith near the middle 
 of the spectrum and falls towards both 
 ends — and the curve representing the 
 process of decomposition of carbonic acid 
 and water in their elements, the so-called 
 assimilation, that main nutrition-process of 
 the plants which takes place through 
 the agency of chlorophyll, coincides almost 
 completely with that light curve. Accord- 
 ing to the later determinations made by 
 Langley, by means of the deflection- 
 spectrum and the bolometer,* the maximum 
 
 *S. P. Langley, Distribution de l’energie dans le spectre 
 solaire-normal. Comptes rendus de l’Academie des Sciences. 
 T. 92. Paris, 1873, p. 701. 
 
54 
 
 THE CLIMATES 
 
 of energy likewise does not lie, as was 
 formerly believed, in the ultra-red but 
 in the yellow, as does the intensity of 
 light in the assimilation of the green 
 parts of plants* I cannot abstain from 
 mentioning this striking coincidence, which 
 might least be expected, considering the 
 character of the material processes. It is 
 one of the most beautiful instances of the 
 adaptation of the organism to its sur- 
 roundings and conditions under which 
 it lives. Under a yellow sun it must 
 certainly be of great advantage in the 
 struggle for existence if yellow is the predom- 
 inant colour and if yellow rays promote 
 
 *Hann (Handbuch des Klimatologie, p. 27) already drew 
 attention to this agreement of the three maxima. 
 
 The curve of the energy of the sun’s spectrum is less 
 steep, left and right of the maximum, than the two other 
 curves; the sensation of brightness and the assimilation are, 
 therefore, not merely functions of the energy of the rays, but 
 also of their wave-length. 
 
OF THE GEOLOGICAL PAST. 
 
 155 
 
 best the nutrition of plants. Accepting 
 this as the cause of the strange phenom- 
 enon, we must likewise consider it very 
 probable, that, at the time when the great- 
 est energy of sunlight belonged to blue 
 and violet rays, these, above all others, 
 must have affected the eye and have caused 
 the strongest assimilation in the vegetable 
 world. 
 
 Therefore, as at that time the true chem- 
 ical rays exercised the greatest influence, 
 those most important processes of life, 
 the vision of animals and the assimilation 
 of plants, could be stronger. Thus, the 
 Pretertiary sun may have accelerated the 
 evolution of the organic world in this way 
 also, that by the quality of its light it 
 fitted the animals for a harder struggle 
 for existence, and produced a more luxu- 
 riant vegetation. 
 
THE CLIMATES 
 
 156 
 
 Further Circumstances in favour of the View 
 that the geological Changes of Climate 
 are to be explained by the History 
 of the Sun’s Evolution. 
 
 Assuming that the sun at a period, geolo- 
 gically speaking, not so remote, was in the 
 condition of a white star, we may look 
 upon and account for the frequent red- 
 blindness as an atavism, a negative inheri- 
 tance from that long time when the eye 
 of our ancestors was not yet sensitive to 
 red rays which were almost entirely want- 
 ing in the white stage of the sun. — The 
 Young-Helmholtz theory of colour sen- 
 sation agrees with the assumption that the 
 maximum of energy in the solar spectrum 
 has been successively, during a long time, 
 in the neighbourhood of the blue-violet 
 (as late as in the Tertiary period); of the 
 middle of the present spectrum (during 
 
OF THE GEOLOGICAL PAST. 
 
 157 
 
 Pre- and Interglacial periods); and of the 
 red (during the Glacial epochs). This 
 theory obtains thereby a genetic and causal 
 foundation. The adaptation for the per- 
 ception of brightness, acquired during 
 each previous condition of sunlight, was 
 successively preserved, being then useful 
 for the distinction of the quality of the 
 light; the originally monochromatic eye 
 became dichromatic, and this afterwards 
 the present trichromatic eye. The colour- 
 perception was developed as an indi- 
 rect result of the changes in the kind 
 of sunlight.* Thus the energy-curves of 
 these different solar spectra correspond 
 completely with those curves which repre- 
 sent the intensity of excitation by the 
 three primary colours. Hence it is also 
 clear why red is the least exciting colour, 
 
 * Consequently we must choose greenish yellow for the 
 middle primary colour. 
 
158 
 
 THE CLIMATES 
 
 why in the dichromatic eye the per- 
 ception of red is most frequently want- 
 ing and why the peripheral extension of 
 the field of vision is smallest for red and 
 greatest for blue, while, on the contrary, 
 in the middle of the retina (because of the 
 yellow colouring around the fovea centralis) 
 all blue and violet becomes somewhat 
 obscurer.* 
 
 According to experiments by Paul Bert 
 and Sir John Lubbock, with Daphnidse, 
 it appears that the eye of even very low 
 animals behaves, on the whole, with regard 
 to colours, in the same manner as the 
 human eye.f 
 
 * More than Goethe himself presumed is there truth in 
 the saying : 
 
 “ War’ nicht das Auge sonnenhaft, 
 
 “ Die Sonne konnt’ es nie erblicken.” 
 
 If in the eye were not a sun, 
 
 It never could behold the sun. 
 
 f Raphael Dubois noted a similar behaviour in the 
 
OF THE GEOLOGICAL PAST. 
 
 159 
 
 A beautiful instance of an indirect adap- 
 tation of the organism to the present kind 
 of sunlight is given by the phospho- 
 rescence of the fire-fly. The strong light 
 emitted by this animal consists nearly 
 entirely of the green and yellow rays of 
 the middle of the solar spectrum ; it con- 
 tains extremely little blue and orange and 
 no red or violet.* That the maximum of 
 energy in this spectrum is situated a little 
 beyond the maximum of energy in the 
 solar spectrum, towards the violet, in the 
 same way as that of the sensitiveness of 
 the eye for rays of different wave-lengths, 
 may possibly be looked upon as an in- 
 heritance from the time when the greatest 
 
 siphons of Pholas dactylus , which, though eyeless, are 
 very sensitive to light. (Comptes rendus de l’Academie des 
 Sciences. Paris, 1889. T. 109. pp. 233 sqq. and 320 sqq). 
 
 *S. P. Langley and F. W. Very, On the cheapest 
 form of light. Amer. Journ. of Science. 1890. (3) Vol 40, 
 pp. 97—113. 
 
THE CLIMATES 
 
 160 
 
 energy of the solar spectrum was due to 
 rays of shorter wave-lengths than at present. 
 
 We find the views set forth here, 
 confirmed to the widest extent in the vege- 
 table world. 
 
 In a similar manner to the colour-per- 
 ception of the eye is, it seems to me, to 
 be explained the predilection of Nature 
 for green in the vegetable kingdom ; 
 namely, as a consequence of adaptation 
 and inheritance. The spectrum of the 
 colouring matter of chlorophyll shows 
 strong absorption in the blue and violet, in 
 the orange and red, less strong in the 
 yellow, very feeble in the green. Therefore 
 this colouring matter in the plants appears 
 green to us. The energy of the rays 
 absorbed by this “ sensibilisatory ” colour- 
 ing matter* is changed into energy of 
 
 * Compare: H. W. VOGEL, Handbuch der Photographie. 
 Part I. 4th Edition, Berlin, 1890, p. 202 sqq. 
 
OF THE GEOLOGICAL PAST. l6l 
 
 a different form, namely not only into 
 chemical action, but partially into fluores- 
 cence and heat, as is proved by the fact 
 that the action exercised by blue rays in 
 the process of assimilation is but rela- 
 tively small, while that of yellow rays on 
 the contrary is the strongest. Though 
 the action of the rays of different wave- 
 lengths, manifesting itself in the chemical 
 change of the molecules, corresponds al- 
 most entirely to the energy of these rays, 
 so that the assimilation and the energy 
 curves, except in their steepness, differ 
 only in that the curve of the assimi- 
 lation is shifted a little towards the red 
 end of the spectrum, yet the blue and 
 violet, the orange and red rays exercise 
 in addition a considerably different action 
 on the molecules. In the same manner 
 as the molecular structure of the pre- 
 sent chromophyll, by whose agency the 
 
l6 2 
 
 THE CLIMATES 
 
 assimilation takes place, has adapted itself 
 to the light of a yellow sun, so — suppos- 
 ing the views advanced in this essay to 
 be correct — it must also have once adapted 
 itself to the light of a sun which was rich 
 in blue and violet rays, and a sun with 
 a preponderance of orange and red rays, and 
 it is natural to consider the strong extinc- 
 tion of just these rays among the 
 present sun-rays as a reminiscence and 
 their present modified actions as physio- 
 logical homologies of the assimilatory pro- 
 cesses of those times. The assimilatory 
 action must always be accompanied by 
 extinction, * and thus we can imagine how, 
 
 * According to the Draper-Vogel “photochemical law of 
 absorption,” that only the rays absorbed by a substance, or 
 by an “optical sensibilisator ” can chemically act upon that 
 substance. (H. W. Vogel, Handbuch der Photographic. 
 Theil i. 4th Edition, Berlin, 1890, p. 58, and J. M. Eder, 
 Die chemischen Wirkungen des Lichtes. Ausfiihrliches Hand- 
 buch der Photographic. 2 nd Ed. Vol. 2. Halle 1891, pp. 
 
OF THE GEOLOGICAL PAST. 1 63 
 
 by a slight modification of the molecular 
 structure, instead of chemical action, work 
 of a different form is effected, and how, 
 when the former decreased, the extinction 
 of the corresponding rays remained, though 
 now mostly caused by fluorescence and 
 heat. In the orange that process of mo- 
 dification seems still to be progressing, and 
 again the relatively strong assimilating 
 action of orange rays is to be considered 
 as a physiological rudiment which re- 
 mained on the other side to that of the 
 colour-perception, probably because here 
 the power of adaptation, and in the eye 
 heredity chiefly prevailed. During the im- 
 measurably long ages when the specific 
 chemical blue rays were predominant the 
 adaptation to these must have become 
 
 153 — 154). — Th. W. Engelmann (Farbe und Assimilation, 
 Botanische Zeitung. Vol. 41. 1883 N os - 1 and 2) has shown 
 its special validity in the process of assimilation. 
 
164 
 
 THE CLIMATES 
 
 very perfect and close, and their assimi- 
 lating action very strong, and during the 
 time of transition to the yellow stage of 
 the sun, when the blue rays were still very 
 abundant, this adaptation must have lasted 
 long, in a similar manner as at present the 
 adaptation to the orange rays. But be- 
 cause of its great perfection and the strong 
 action of the relative rays, adaptation to 
 the then predominant rays between blue 
 and yellow would have been useless, and 
 this has never, therefore, taken place in a 
 noticeable degree. 
 
 Hence the present world owes the beau- 
 tiful emerald colour of Flora’s garment to 
 these great changes of solar radiation.* 
 
 * Likewise the red colour of the young leaves of many 
 plants has probably to be looked upon as the transitory 
 and imperfectly inherited condition from Preglacial time. At 
 present the red colouring matter takes no part in the making 
 of fecula ; but, according to chemical experiments, still 
 in the making of sugar. 
 
OF THE GEOLOGICAL PAST. 1 65 
 
 The connection of these with a great 
 morphological change of the vegetable 
 world seems to me to be quite certain. 
 The chief plant types of the Palaeozoic 
 age — the Calamites, Lepidodendra, Sigil- 
 lariae — as well as the Equisetaceae, 
 tree ferns, Cycads and Conifers of the 
 Mesozoic period, were all sparingly 
 branched and the total superficial extent 
 of their leaves was but small, as is likewise 
 the case with all existing lower (that is 
 old typical) chlorophyll-plants. Among the 
 Angiosperms the Casuarinae ( Chalazogamae ), 
 which according to Treub’s important inves- 
 tigations,* are equivalent to the Mono- 
 and Dicotyledons together ( Porogamae ), 
 and, therefore, are originating from early 
 
 * M. Treub, Sur les Casuarinees et leur place dans le 
 systeme naturel. Annales du Jardin Botanique de Buiten- 
 zorg. Vol. 10. Batavia 1891, pp. 145 — 231, with 21 
 Plates. 
 
THE CLIMATES 
 
 166 
 
 Mesozoic time, have but very poorly 
 developed organs of assimilation. Although 
 during the Cretaceous period, and even 
 during the Jurassic period, plants existed 
 more richly branched and with a larger 
 total area of leaf-surface (Dicotyledons), 
 these formed only a very inconsiderable 
 part of the vegetation of the earth. It was 
 not before the Tertiary period that, starting 
 from the north pole, their centre of develop- 
 ment, they drove entirely into the background 
 the other plant classes. The small extent of 
 leaf surface of the Pretertiary chlorophyll 
 plants can only be explained by the relatively 
 stronger assimilation, since the supposition 
 of a formerly greater amount of carbonic 
 acid in the atmosphere can now no longer 
 be seriously discussed; increased transpi- 
 ration certainly did not exist in the damp 
 climates of these periods, and simple 
 adaptation of organs so very important to 
 
OF THE GEOLOGICAL PAST. 1 67 
 
 life would certainly have been brought 
 about in the vegetable mantle of the earth 
 long before the Tertiary period, had it 
 been necessary. 
 
 Thus, as well in the past of the vegetable 
 world as in that of the animal world,* 
 the majestic effect of that limitation ot 
 the solar energy bestowed on our planet 
 becomes apparent, and we see this very 
 sparingness of the sun become the cause 
 of the evolution of the most perfect and 
 highest forms of life. But the whole 
 history of its evolution has been stamped 
 by the brilliant orb of day in every green 
 leaf and in the sense of sight of animals. 
 
 * Compare especially pp. 55 — 59. 
 
 THE END.