4*20 Wortman — Studies of Eocene Mammalia in the 



Just what climatic conditions obtained in the polar regions 

 prior to the Cretaceous, whether there were alternate periods 

 of heat and cold with consequent glaciation, or whether the 

 climate was uniformly tropical, are questions with which our 

 present inquiry is not vitally concerned, for the reason that 

 the origin of the higher forms of mammalian life can not be 

 consistently placed at a much earlier date than the beginning 

 of the Cretaceous. 



Touching the question of former glacial epochs, however, it 

 is proper to state that more recent geological investigations 

 have shown the existence of boulders of great size and thick- 

 ness, scratched and glacial -like, in beds of Permian and prob- 

 ably of Triassic age also, in Australia, New Zealand, South 

 Africa, India, and presumably in South America. The wide- 

 spread occurrence of these phenomena over the Southern 

 Hemisphere would seem to be impossible of explanation on 

 any hypothesis other than that of glaciation or a period of 

 cold. Such facts have led many geologists to believe in the 

 existence of successive or intermittent glacial epochs in the 

 past. A fair estimate of present opinion on the subject may 

 be gained from Dana's statement,* which is as follows : " Thus, 

 throughout the earth's history since life began, the only cold 

 epochs of which proof has been found occurred near or at the 

 close of the Permian, at the close of the Triassic, and during 

 the Glacial period. At the close of the Cretaceous, another 

 epoch is suspected to have occurred, but without direct evi- 

 dence." 



With a few exceptions the evidence of glaciation in the 

 Northern Hemisphere is confined almost exclusively to the 

 Glacial epoch proper, and it is to be seriously doubted whether 

 conclusive proof of wide-spread glacial action in any former 

 period will ever be found. At all events we are not warranted 

 in the belief that a frigid temperature overspread the Northern 

 Hemisphere at any time between the Jurassic and the Tertiary. 

 On the contrary, all the testimony which has yet been gathered 

 from various available sources shows that there were slow and 

 gradual changes of temperature from tropical, through all the 

 stages of subtropical, warm temperate, temperate, cold tem- 

 perate, subfrigid, and frigid, in the long interval between the 

 Jurassic and the Glacial epochs. 



The class of facts upon which we most largely depend for 

 conclusions respecting the former climatic conditions in any 

 region are furnished by the fossil remains of plants and animals 

 imbedded in its strata. In order that evidence of this character 

 may become available, we must assume two propositions : (1) 

 * Manual of Geology, 1894, p. 1027. 



