30 LOST AND VANISHING BIRDS 



has been relatively so recent. This apparent 

 anomaly admits of a very easy explanation. 

 Islands that have from a variety of causes, which 

 we need not here stay to discuss, remained in a 

 state of great isolation, are generally found to be 

 inhabited by a fauna, or the relics of a fauna once 

 more widely dispersed, or have developed a variety 

 of species by the aid of their long-enduring isola- 

 tion from all allied forms. It thus happens that 

 these remotely isolated spots have gradually be- 

 come possessed of a fauna more or less peculiar 

 to themselves, species being found on them that 

 are not found anywhere else. But, on the other 

 hand, islands that are not so isolated, either being 

 situated close to continents, of which it is certain 

 they formed a geologically recent part, or are 

 located in seas in which uninterrupted intercom- 

 munication with the nearest land masses is main- 

 tained by normal migration across them, or the 

 various fortuitous methods of dispersal, have few 

 or no such opportunities for establishing a peculiar 

 fauna, and consequently preserve their biological 

 homogeneity. The British Islands are a capital 

 example of the latter class of islands, and their 

 avifauna is almost exactly identical with that of 

 the adjacent continent, and is subject to very 



