532 



GENERAL CONSIDERATIONS 



a temperature little above freezing, no light, and no movement of the 

 water sufficient to produce a current. 



578. Island Faunas. — The fauna of an island depends upon whether 

 or not the island is what is known as a continental one, adjacent to a 

 continent, with which in the past it may have been in communication. 



180 



160 



140 120 100 80 60 40 20 



180 160 140 120 too 80 60 40 



Fig. 368. — The zoogeographical regions of the western hemisphere. 



or an oceanic island, lying at a distance from any continent and with no 

 such past connection. Frequently the faunas of continental islands 

 are similar to those of the adjacent mainland, even the same amphibians 

 — which never occur in the ocean — being found, as well as mammals to 

 which even a narrow channel would be a barrier. Often characteristic 

 types are absent from such islands, as are snakes, for instance, from Ire- 



