CAUSES OF THE EVOLUTION AND EXTINCTION OF THE TITANOTHERES 



861 



Conclusions. — Since there is little parallel between 

 the Eocene changes in Europe and those of western 

 America, and a very close parallel between the phe- 

 nomena of extinction in Europe and those of western 

 America, the conclusion is inevitable that physio- 

 graphic changes are not directly potent factors of ex- 

 tinction but are indirectly potent through the biotic 

 changes which they induce, new types of adaptation, 

 new forms of competition, etc. 



CHANGES OF CLIMATE 



We have now to consider changes in temperature 

 and moisture as brought about by geologic and physio- 

 graphic changes and as- effecting in turn biotic 

 changes — changes in the fauna and flora. 



Pleistocene secular increase of cold. — The effects of 

 secular lowering of temperature must be analyzed with 

 some care. At first sight the original theories of 

 extinction of Buffon and Cuvier regarding refrigeration 

 or direct action of cold are very simple and plausible, 

 but on examining all the instances of extinction which 

 occurred during the Pleistocene or glacial epoch we 

 find that this simple or obvious explanation is not the 

 true one. To take the glacial epoch, which affords 

 the chief illustration, it is more in general accord with 

 the facts to say that this period originated certain new 

 conditions of life which diminished numbers and 

 hastened extinction. These conditions include such 

 phenomena as deforestation, enforced migrations, 

 overcrowding, changes of food, unfavorable conditions 

 of mating and reproduction, new relations to enemies, 

 and other indirect results. The reason refrigeration 

 can not be considered alone or as a direct cause lies in 

 the remarkable powers of plastic adaptation which 

 many mammals, including man, exhibit to secular or 

 regional lowering of temperature. The horse of North 

 America became extinct during the glacial epoch, 

 yet there is every reason to believe that its extermina- 

 tion was not directly connected with the Ice Age. 



General phenomena. — Among the observed climatic 

 phenomena and biotic effects of imusual cold periods 

 are the following: Harsh or unusual conditions of life 

 caused by snowstorms, blizzards, ice floods, diminu- 

 tion of food supply, limited choice of food caused by 

 change of flora and deforestation, enforced choice of 

 deleterious food, changes in fertility and reproduction 

 rate, dangers to young, arrested growth, diminution 

 in number causing diminished herds, enforced migra- 

 tions, crowding southward. 



Protective adaptation. — Secular cold is very slowly 

 progressive and has generally been accompanied step 

 by step by progressive adaptation among the mammals 

 to resist cold. Resistance depends upon (a) internal- 

 heat producing power, which is a progressive adapta- 

 tion of the ascending series of mammals as distinguished 

 from the reptiles; (&) acquisition of a warm external cov- 

 ering of wool and hair; (c) development of subcutane- 



ous and other fatty layers; (d) power of hibernation. 

 The well-known instances of adaptation to extreme 

 cold among the Proboscidea (Elephas primigenius, 

 woolly mammoth. Mastodon americanus, mastodon), 

 rhinoceroses {R. ticliorhinus, woolly rhinoceros), horses 

 (E. przewalsTci), northern ruminants (Saiga antelope, 

 Bactrian camel, Barren-Ground and Arctic caribou, 

 musk ox) indicate again that we must not assume that 

 refrigeration was a direct or sole cause of extinction. 



Glacial and postglacial extinction. — Wallace observes 

 (1876.1, p. 151): 



We have proof in both Europe and North America that just 

 about the time these large animals were disappearing all the 

 northern parts of these continents were wrapped in a mantle 

 of ice; and we have every reason to believe that the presence 

 of this large quantity of ice (known to have been thousands of 

 feet if not some miles in thickness) must have acted in various 

 ways to have produced alterations of level of the ocean as 

 well as vast local floods, which would have combined with the 

 e.xcessive cold to destroy animal life. 



And again (1881.1, p. 117): 



We can therefore hardly fail to be right in attributing the 

 wonderful changes in animal and vegetable life that have 

 occurred in Europe and North America between the Miocene 

 period and the present day, in part at least, to the two or more 

 cold epochs that have probably intervened. These changes 

 consist, first, in the extinction of a whole host of the higher 

 animal forms; and, secondly, in a complete change of types 

 due to extinction and emigration, leading to a much greater 

 difference between the vegetable and animal forms of the 

 Eastern and Western hemispheres than before existed. 



Wallace's views regarding glacial extinction are 

 modified by the present four-glaciation theory. The 

 large Afi-ican-Asiatic fauna of Europe, namely, the 

 hippopotami, the rhinoceroses, the elephants, and the 

 mammoths, survived the first, second, and third 

 glaciations but perished before or during the extremely 

 severe climatic conditions of the fourth glaciation. 



Freeh observes (1906.1): 



The chief problem in regard to extinction is in general 

 confined to the question whether the great climatic changes of 

 former geological periods coincide with the great transforma- 

 tions of the plant and animal kingdoms. Corresponding with 

 the fall in temperature at the close of the Tertiary in Europe 

 is the disappearance of all the tropical and subtropical animals 

 (tapirs, mastodons, hipparions, and the last tropical antelopes, 

 Protragelaphus and P alaeoryx) . In the Quaternar}^, before the 

 advent of the [last] glacial period the last remnants of the sub- 

 tropical fauna vanished from central Europe (Hippopotamus 

 major, M achaerodus) also the tropical elephants and rhinoc- 

 eroses characterized like the recent forms by a practically 

 hairless skin. The mammoth and the Arctic rhinoceros had 

 in their long and dense woolly coat perhaps the best protection 

 against cold that any terrestrial animal has ever possessed- 

 The adaptations to cold, equally well defined in large and small 

 animals of the circumpolar fauna, presuppose a long period of 

 time for their development. The mammoth probably existed 

 in Siberia under conditions of temperature similar to those 

 prevailing at the present time, before the beginning of the 

 glacial period. The general fall in temperature was probably 

 first marked in northern Siberia and thus it was here that 

 conditions first became favorable for the evolution of an Arctic 

 fauna. The general drop of 4° C. in temperature moved the 



