June 19, 1890] 



NATURE 



^77 



and warm terrestrial climate from the equator to the 

 poles. Geographically we have sharply contrasted floras, 

 and we have moreover widely distributed deposits, in the 

 formation of which great masses of ice must have played 

 a part, and thus the old views are utterly overthrown. But 

 when we go further, and seek to learn from the facts before 

 us what the conditions really were, we are quickly 

 admonished that our knowledge is as yet far too small to 

 admit of any definite representation of these conditions. 

 We may say with much probability that the differences 

 of the floral regions must be ascribed to differences of 

 climate, and that, locally, the temperature was so low as 

 to allow of the formation of great masses of ice ; but any- 

 thing beyond this is quite uncertain, and no one of the 

 assumptions that have been made to explain the con- 

 ditions of that epoch has any claim to validity. Those 

 early ages present us with so much that is strange to us, 

 the unknown is so vast in comparison with what we know, 

 that we dare not as yet attempt any generalization of our 

 knowledge. 



We pass over the formations which succeed the Coal- 

 measures, viz. the Permian, the Trias, the Jura, and the 

 Chalk, and after this enormous interval we turn our 

 attention to Tertiary times. Here begin those modern 

 developments that have resulted in our present world ; 

 the chief types of animals and plants are the same as 

 those of our own day ; and it is only since the beginning 

 of Tertiary times that mammals predominate among the 

 fauna of the land, whereas in the previous formations 

 this leading part had been played by reptiles. 



At that time Europe was far more cut up by inland 

 seas than it now is, and formed a dismembered assem- 

 blage of islands and peninsulas. In the first division of 

 the Tertiary age, the Eocene, the seas around its coasts 

 were tenanted by animals of a tropical character. In the 

 later subdivisions, this character was gradually lost. In 

 the Oligocene, a marine fauna of a tropical character 

 extends only to a line which about coincides with the 

 northern limit of the Alps. In the Miocene, which next 

 follows, the fauna even of this part of Europe is, at the 

 utmost, sub-tropical ; and, by degrees, the forms which 

 give evidence of a warm climate gradually diminish, so 

 that towards the end of the last division, the Pliocene, the 

 conditions were almost the same as to-day. 



What we know of the land organisms agrees entirely 

 with these indications afforded us by the marine fauna, 

 at least in their leading characteristics, since we equally 

 find, at the beginning of Tertiary times in Europe, a 

 predominance of sub-tropical and tropical types, which, 

 later on, were replaced by a flora representative of a 

 temperate climate. In detail, indeed, there are many 

 and not unimportant deviations. Thus, for instance, the 

 flora the remains of which are preserved in the calcareous 

 tufa of Sezanne in Champagne, or in the marls of 

 Gelinde, belongs to the Lower Eocene. The forms here 

 represented are such as at the present time are pecuHar 

 to the southern part of the temperate or the sub-tropical 

 zone ; numerous evergreen oaks, laurels, cinnamon and 

 camphor trees, various Myrtaceae, Araliaceae, figs, mag- 

 nolias, &c.; many forms point decidedly to a tropical 

 climate, but among them we find also, walnut trees, limes, 

 alders, willows, ivy, and vines, which have an opposite 

 character. Palms and cycads, the specially characteristic 

 forms of hot climates, are absent, or at any rate have not 

 been detected. On the whole, botanists are inclined to 

 infer for that epoch in Central Europe such a climate 

 as now obtains in Southern Japan in 33° N. latitude. 



We meet first with truly tropical floral characters in 

 somewhat later deposits, viz. in the Middle and Upper 

 Eocene. At that time there flourished on the mainland 

 and islands of Europe great palms and a number of other 

 plants, whose nearest relatives now exist in tropical Africa, 

 India, and Australia. To judge from the land flora, there 

 was then a maximum of warmth in our neighbourhood 



NO. 1077. VOL 42] 



(Vienna), from which up to the end of Tertiary times a con- 

 tinuous fall took place. In the Oligocene and Lower 

 Miocene the prevailing character is still that of a tropical 

 or sub-tropical region, but the number of forms that now 

 live in temperate regions has considerably increased ; 

 such as now live in Australia occur in remarkable quantity. 

 Then in the Upper Miocene of Central Europe we meet 

 with a flora such as at the present day characterizes the 

 warmer parts of the temperate zone, and in which forms 

 allied to the present flora of North America are especially 

 prominent. In the Pliocene, the latest subdivision of the 

 Tertiaries, the change has progressed still further, and at 

 its end we find in our neighbourhood an assemblage of 

 plants nearly recalling that of the present day, with but a 

 slight intermixture of those of warmer regions. 



We may grant generally that these facts prove the 

 existence in Tertiary times of a warmer climate than 

 now prevails in Europe, even though there may be great 

 differences of opinion as to the amount of the difference. 

 Heer, to whom we are indebted for the most important 

 investigations of this subject, has endeavoured to deter- 

 mine the mean annual temperature at certain definite 

 geological epochs from the characters of their respective 

 floras. He found that on the northern border of the Alps 

 in Switzerland, at the epoch of the Upper Ohgocene, 

 there was a mean temperature of between 20° and 22"" C. 

 (68°-72'' F.), such as at the present day is that of Cairo, 

 Tunis, Canton, or New Orleans ; at the time of the Upper 

 Miocene, one of 18° or 19° C. (64°-66° F.), corresponding 

 to that of Messina, Malaga, Madeira, and Nagasaki ; 

 whereas at the present time the mean annual temperature of 

 Zurich is 8°73 (47°7 F.), that of Geneva 9°-67 C. (49°"4 F.). 

 But whereas Geneva and Zurich now lie high above sea- 

 level, we have proofs that in Tertiary times the sea-level 

 was much higher in that neighbourhood than now, there- 

 fore that this flora grew at a small height above the sea, 

 which would imply alone an increase of about 3° C. (5^° F.) 

 of temperature. It follows, then, that at the time of the 

 Upper Oligocene the temperature was about 9° C. (16° F.), 

 in that of the Upper Miocene about 7"" C. (12° F.) higher 

 than at present. 



With respect to these figures, we rnust, however, bear in 

 mind that in such computations no allowance is made 

 for the acclimatization of species and whole genera in the 

 course of long geological periods, and therefore that the 

 assigned variations of temperature are almost certainly 

 00 high. Moreover, we must remember that, at that time, 

 tEurope was far more than now interpenetrated by inland 

 seas and straits, and therefore that its climate was more in- 

 sular, the summers being cooler and the winters warmer 

 than now. But whatever weight we give to these con- 

 siderations, they are alone insufiicient to account for the 

 whole of the difference between the Eocene and the 

 present floras. We must perforce admit that other and 

 deeper-lying causes have co-operated in producing the 

 observed differences. 



The examination of the Tertiary floras of high northern 

 latitudes leads us very decisively to a similar conclusion. 

 The various English, American, Danish, and especially 

 the Swedish expeditions have discovered in numerous 

 localities the Tertiary plant-remains of the polar regions, 

 the floras of which have been worked out by Heer. 

 Places which are now among the coldest known spots of 

 the earth have yielded the remains of a rich forest vegeta- 

 tion ; nay, within the polar circle itself are found plants 

 which at the present time find even our own latitudes 

 too cold for them. The most northern point from which 

 we have plant impressions is Grinnell Land in the North 

 American archipelago, in 8r45' N. lat. Its present mean 

 annual temperature is about - 20° C. (4° F.). The flora con- 

 sists chiefly of conifers, among which are our common 

 pine, two species of fir, and the American swamp cypress 

 ( Taxodium distichuni) ; with these are associated elms, 

 limes, birches, poplars, hazel, and some others, the 



