504 



GENERAL CONSIDERATIONS 



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

 water sufficient to produce a current. 



576. 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 



ISO 160 140 120 100 80 60 40 20 



Fig. 310A. — 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- 



