ZOOGEOGRAPHICAL REGIONS 211 
is undoubtedly the sea, for a broad channel offers an 
insuperable barrier to the passage of most land 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 life 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 life 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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