CENOZOIC ERA: AGE OF MAMMALS 639 



occur. These Australian mammals are very different from those of 

 other parts of the world, but are related to those that lived in Europe 

 in the Mesozoic ; types which with a few exceptions have long since 

 been extinct in other continents. The natural inference is that Aus- 

 tralia was isolated during the whole of the Tertiary epoch and that, 

 because of the non-interference of the higher mammals, the mar- 

 supials have been able to develop there along their own lines, produc- 

 ing the kangaroo, the wombat, and the other animals peculiar to that 

 continent. 



The effect of isolation and migration on animal life is especially 

 well shown in the history of Tertiary mammals. 



Eocene Invasion. — The first, and perhaps most important 

 Tertiary migration occurred at the very beginning of that epoch, 

 as is shown (1) by the sudden appearance of true mammals which, 

 though simple in structure, were already somewhat diversified, and 

 (2) by their similarity in all parts of the world where found. The 

 land connection, or connections, which permitted this migration 

 from a center whose location is at present unknown, disappeared, 

 perhaps by subsidence, and the continents of the Old and the New 

 World were apparently again separated by broad seas for a long 

 period of time (Fig. 561), permitting the life of the isolated regions to 

 develop independently during the Eocene. A comparison of the life 

 of the Middle and Upper Eocene shows that the odd-toed, hoofed 

 mammals (perissidactyls) of Europe and North America differed in 

 many respects, and that, although horses developed on the two conti- 

 nents, they were markedly dissimilar. The same dissimilarity is shown 

 in the carnivores and rodents. During this period of isolation three 

 new families made their appearance in America, the camels (p. 615), 

 oreodonts (p. 619), and armadillos (p. 621) ; and the most striking 

 of the Rocky Mountain Eocene mammals (Amblypoda) were prob- 

 ably extinct in Europe before the Upper Eocene, but did not become 

 extinct in America until near the close of the Upper Eocene. The 

 resemblance that existed between the mammals of the two continents 

 is only that of descent from similar ancestors. 



Oligocene Invasion. — The simultaneous appearance in Europe 

 and North America (Fig. 562) of new families of mammals of a de- 

 cidedly more modern type than those of the Eocene, points strongly 

 to their evolution in some region separated from both continents for 

 a long period, and united to them at approximately the same time by 

 renewed land connections. It should not be inferred from the above 



CLELAND GEOL. — 41 



