62 A HISTORY OF BIRDS 



causes — temperature, food and physical peculiarities. Thus 

 Woodpeckers, for example, will be confined to the forests, Bus- 

 tards to the dry plains, Grebes and Divers to the lakes. In 

 other words, any given species is to be sought for only in suit- 

 able spots within the distributional area, and these spots are known 

 as the "stations" of the species within that area. Thus a clear 

 distinction is to be drawn between the area of distribution — 

 the " locality" or '-habitat" of the species — and its "station". 

 This last, as we have already hinted, is more often represented 

 by a series of discontinuous areas — or isolated stations — than 

 by one continuous tract. 



Passing now to some of the more general peculiarities of 

 distribution it is to be remarked that though, in surveying the 

 habitat of any particular species one may expect to find that 

 species in more or fewer numbers wherever suitable conditions 

 occur, this expectation is by no means always realised. The 

 curiously local distribution of birds like the Nightingale, Red- 

 start and Stonechat, for exam.ple, among our British birds 

 well illustrate this point, since these birds may be tolerably 

 abundant in one place and yet appear to shun areas in the im- 

 mediate neighbourhood which seem in every way suitable to 

 their needs. 



More remarkable still are those cases where the " stations " 

 are not so many isolated spots dotted over a continuous area, 

 or zoological province, but are separated by enormous distances, 

 are scattered, in short, over one or more separate zoological 

 provinces. These cases of "discontinuous distribution" are of 

 peculiar interest, for they show, in the first place, that, in the 

 majority of cases, species so isolated must have become so by 

 the extinction of more or fewer intermediate " stations ". Such 

 widely sundered sanctuaries, so to speak, as isolation areas, 

 play an even more potent part in the evolution of species than 

 do stations dotted over a continuous zoogeographical area. But 

 of this more anon. 



Thus, then, it has come to pass that the Ornithologist, 

 in studying the birds of the world — as one might study the 

 people of the world — ignores political boundaries, and divides 

 the surface of the earth according to the dominant and peculiar 

 types of birds which are characteristic of such and such areas. 

 The governing factors in this distribution are many — climate, 



