ZOOGEOGRAPHICAL REGIONS 211 



is undoubtedly the sea, for a broad channel offers an 

 insuperable barrier to the passage of Inost lajid animals, 

 save those small enough, or resistant enough, to be 

 carried passively by winds and waves or an animal 

 host. Here, obviously, we approach difficult questions. 

 Geological research, especially in recent years, has 

 familiarized geographers with the idea that great 

 changes in the distribution of land and water have 

 taken place on the surface throughout geological time. 

 Now to consider this distribution in its relation to 

 animal Hfe and evolution from the Cambrian period 

 onwards would obviously be a Herculean task, though 

 it is one which has been attempted, e.g. by Arldt. 

 Without such a consideration, however, we cannot 

 hope to explain in its entirety the distribution of 

 animals at the present time. But without attempting 

 to do this, it is of interest to note the main features 

 of the distribution of the higher forms of Hfe in the 

 existing continental masses. As we saw in the Intro- 

 duction, such a division of the globe into zoogeo- 

 graphical regions is primarily of'interest to the zoologist 

 rather than to the geographer. This is especially true 

 when the subject is treated in detail, as in most of the 

 works on the subject. At the same time for historical 

 reasons, if for no other, the subject is too important 

 to be entirely omitted, and in this concluding chapter we 

 shall consider briefly the classical ' regions ' into which 

 the globe has been divided by zoologists, on the basis 

 especially of the distribution of birds and mammals. 



In dividing up the globe in this way the first point to 

 be emphasized is that, though the geographers are accus- 

 tomed to lay great stress upon the distinction between 

 the American and Eurasiatic continents, between 

 the Old and the New Worlds, no such distinction 



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