26 LAND MAMMALS IN THE WESTERN HEMISPHERE 



hasty inferences have often led to contradictory and absurd 

 conclusions. When properly employed, the fossils give a more 

 continuous and complete history of climatic changes than can, 

 in the present state of knowledge, be drawn from a study 

 of the rocks alone. For this purpose plants are particularly 

 useful, because the great groups of the vegetable kingdom are 

 more definitely restricted in their range by the conditions of 

 temperature and moisture than are most of the correspondingly 

 large groups of animals. Not that fossil animals are of no 

 service in this connection ; quite the contrary is true, but the 

 evidence from them must be treated more carefully and criti- 

 cally. To illustrate the use of fossils as recording climatic 

 changes in the past, one or two examples may be given. 



In the Cretaceous period a mild and genial climate prevailed 

 over all that portion of the earth whose history we know, and 

 was, no doubt, equally the case in the areas whose geology 

 remains to be determined. The same conditions extended 

 far into the Arctic regions, and abundant remains of a 

 warm-temperate vegetation have been found in Greenland, 

 Alaska and other Arctic lands. Where now only scanty and 

 minute dwarf willows and birches can exist, was then a luxuri- 

 ant forest growth comprising almost all of the familiar trees of 

 our own latitudes, a most decisive proof that in the Cretaceous 

 the climate of the Arctic regions must have been much warmer 

 than at present and that there can have been no great accumu- 

 lation of ice in the Polar seas. Conditions of similar mildness 

 obtained through the earlier part of the Tertiary. In the 

 Eocene epoch large palm-trees were growing in Wyoming and 

 Idaho, while great crocodiles and other warm-country reptiles 

 abounded in the waters of the same region. 



It is of particular interest to inquire how far the fossils of 

 Glacial times confirm the inferences as to a great climatic 

 change which are derived from a study of the rocks, for this 

 may be taken as a test-case. Any marked discrepancy be- 

 tween the two would necessarily cast grave doubt upon the 



