366 GENERAL ZOOLOGY 



same group have their natural habitats in widely separated parts 

 of the world. Thus the tapir is found in Central and South 

 America and in Southern Asia; and alligators occur in central 

 China and Southeastern United States. Why have many ani- 

 mals failed to occupy certain regions favorable for their support 

 and why are other groups found in widely separated regions? 



In answering these questions, in addition to the usual factors 

 operating in relatively stable communities and in ordinary 

 ecological succession, the effects of major geographical changes, 

 dealt with in geology and paleontology, must be considered. 

 Paleontology demonstrates that animal groups often originated 

 in localities far removed from the present habitats of the living 

 descendants. Fossils show that members of the camel family 

 formerly ranged North America as well as Asia, a fact that can 

 be readily understood when it is pointed out that these two 

 continents were formerly connected where Bering Strait now 

 separates them (Figs. 201 and 202). Ancestral camels became 

 extinct in North America but survived in Asia, whence they 

 spread to Africa. In South America they are represented by the 

 llama. The horse is not indigenous to North America. It arose 

 there, spread to the Old World, but became extinct in the place 

 of its origin during the Quaternary period. In the Old World it 

 survived and gave rise to modern horses. Paleontology discloses 

 the wandering of many animals from place to place, in all prob- 

 ability in search of food and shelter, until the paths of these 

 various migrations have become a complicated network, which 

 has been interpreted in relatively few cases by fossil remains. 



Discontinuous distribution in many cases is the result of the 

 creation of barriers isolating animal communities in whole or in 

 part in various ways. The peculiar fauna of Australia is undoubt- 

 edly due to the fact that this continent has been separated from 

 other continents from the beginning of Cenozoic times, with the 

 result that its egg-laying mammals are entirely different from 

 mammals found in continental Asia. Accordingly, prior to the 

 coming of civilized man much of its fauna remained mesozoic in 

 character. Apparently the higher types of mammals never 

 evolved in Australia. In addition to water, other barriers 

 such as mountains, hills, deserts, temperature, climate, etc., have 

 been equally effective in bringing about a checker-board dis- 

 tribution of many animals of similar species. 



