194 THE STORY OF BIRD-LIFE. 



work. Those who are interested in this fas- 

 cinating subject should consult the works of 

 Dr Sclater, Mr Alfred Eussel Wallace, Prof. 

 Newton, or Dr Sharpe. 



The basis upon which these boundaries rest 

 is their resident or non-migratory population. 

 For, besides which, there is a floating popula- 

 tion, which periodically leaves one place of 

 residence, often a high northern latitude, for 

 another often not far removed from, or even 

 within, the tropics. Concerning these migratory 

 tribes we shall have more to say later. 



The study of the resident populations then — of 

 birds or otherwise — of each of these great 

 regions is one of extreme interest and import- 

 ance. Not because it tells us that such and 

 such a bird or bea&t is found in such and such 

 a region, but because we learn from it much 

 concerning the past history of the earth that 

 would otherwise never have been suspected. 

 Zoo-geography may be regarded as a sort of 

 hand-maid to geology. 



For instance, when we come to study the 

 fauna of the British Islands, and to compare it 

 with that of the European continent, we find 

 that the two are almost identical, though the 

 further we get away, the more emphatic becomes 

 the difference. 



That the same species of birds occur in both 

 areas does not seem strange, for we know that 

 they can easily pass, what is to us an insurmount- 

 able barrier, the sea that divides us from the 

 continent. But what about the mammals and 

 reptiles, the hare, the fox, the hedgehog and 



