442 COMPARATIVE ZOOLOGY 



times. Thus, Australia has always been the home of 

 marsupials, and South America of edentates. 



It is a general rule that groups of distinct species are 

 circumscribed within definite, and often narrow, limits. 

 Man is the only cosmopolitan ; yet even he comprises 

 several marked races, whose distribution corresponds 

 with the great zoological regions. The natives of Aus- 

 tralia are as grotesque as the animals. Certain brutes 

 likewise have a great range : thus, the puma ranges 

 from Canada to Patagonia; the muskrat, from the 

 Arctic Ocean to Florida; the ermine, from Bering 

 Strait to the Himalayas ; and the hippopotamus, from 

 the Nile and Niger to the Orange River. 178 



Frequently, species of the same genus, living side 

 by side, are widely different, while there is a close re- 

 semblance between forms which are antipodes. The 

 mud eel of South Carolina and menobranchus of the 

 Northern States have their relatives in Japan and Aus- 

 tria. The American tapir has its mate in Sumatra, the 

 llama is related to the camel, and the opossum to the 

 kangaroo. 



The chief causes modifying distribution are tempera- 

 ture, topography, ocean and wind currents, humidity, 

 and light. To these may be added the fact that ani- 

 mals are ever intruding on each other's spheres of exist- 

 ence. High mountain ranges, wide deserts, and cold 

 currents in the ocean are impassable barriers to the 

 migration of most species. Thus, river fish on opposite 

 sides of the Andes differ widely, and the cold Peruvian 

 current prevents the growth of coral at the Galapagos 

 Islands. So a broad river, like the Amazon, or a deep, 

 narrow channel in the sea, is an effectual barrier to 

 some tribes. Thus, Borneo belongs to the Indian region, 

 while Celebes, though but a few miles distant, is Aus- 

 tralian in its life. The faunae of North America, on 



