CENOZOIC ERA. AGE OP^ MAMMALS. 403 



2. Migrations during' the Glacial Epoch and 

 their Effect on the Geographical Distribution of 

 Organisms. The oscillations of the earth's crust during 

 glacial times produced great changes in Physical Geog- 

 raphy, elevation enlarging and subsidence' diminishing 

 the area of the continents. In this manner gateways 

 were opened permitting migrations from one conti- 

 nent to another, as for example between North America 

 and Asia through Bering Straits, and between Europe 

 and Africa through the Mediterranean. Again, the great 

 changes of climate from subtropical mildness to extreme 

 arctic rigor, and back again to temperateness, enforced 

 migrations southward and northward, perhaps several 

 times. These migrations, whether permitted or enforced, 

 produced a mixing of different faunas and flotas on the 

 same gm*L(l ; and the severe competitiv^/ struggles 

 amon^themAtggether with the great chajig^ of climaty 

 conditions, caus&Ljiiany charige#r^0artlv by extinction 

 tnd partly /by/ modification. After these migrations, 

 minglings, struggles, and consequent modifications/ the 

 resulting faunas and floras were again in\ma#y cases 

 reisolate^m their new homes by subsidence. In these 

 :ed new homes they have undergone slow changes 

 by evolution until Ahe present time. Thus have come 

 about the present Geographical faunas spoken of in chap- 

 ter iii., section 4, o*f Part I. (P. 118). Now, as the Glacial 

 epoch is a comparatively recent geological event, it is 

 evident that the migrations of that time furnish a key 

 to the present distribution of organisms ; and conversely, 

 the present distribution of organisms is a key to direction 

 of migrations during that time. We give a few striking 

 examples illustrating this very interesting subject, and 

 completing the explanations given in Part I. 



1. Alpine Species. It is a curious fact that alpine 

 species of plants and insects (i. e., species which live on 

 mountains near the snow line) are very similar in all parts 



