412 GENERAL ZOOLOGY. 



regions on land, but on our Atlantic coast we can set the 

 boundaries at approximately Cape Cod and Cape Hat- 

 teras; the Arctic fauna extending down to the former; 

 the tropical, though not so distinctly, up to the latter 

 promontory. Besides these coast areas there exists a 

 pelagic fauna composed of animals which live on the 

 surface of the high seas, and another abyssal fauna con- 

 taining those in the greater depths (500 fathoms and 

 more) of the sea. 



These different fauna9 suggest numerous problems, only 

 one or two of which can be alluded to here. In part the 

 differences between them are easily explicable upon 

 climatic grounds, but there are numerous others which 

 are not solved so easily. Why are the Marsupials re- 

 stricted to America and the Australian region? Why 

 are all the ostrich-like birds, except the ostrich itself, 

 confined to the southern hemisphere? How is the pe- 

 culiar distribution of the Dipnoan fishes (p. 334) to be 

 explained? Clearly these are not the result of climate. 

 Not all these questions have been answered, but the solu- 

 tion of many is found in the study of the past history of 

 the earth when it is found that once North America and 

 the Old World were connected and that once Marsupials, 

 for instance, occurred in Europe as well. A knowledge 

 of this past history shows that the later the connection 

 between two regions of like climatic conditions the more 

 close their faunae are related to each other, while those 

 which have been separated for a greater length of time 

 are much more distinct, for their inhabitants have had 

 a longer opportunity to develop in their own lines. 



