200 THE MIGRATION OF BRITISH BIRDS 



segregated into a well-marked species. It is very- 

 probable that the Nuthatch of Algeria will be found 

 to differ from the European type. It is therefore 

 most significant that the only instance of an endemic 

 species in these Mediterranean migration-swept islands 

 is that of one whose parent form is non-migratory, and 

 thus never interfered (by the intermixture of individuals) 

 with the process of differentiation. 



Passing now to the Canary Islands, w'e are confronted 

 with a very different series of facts. As we have already 

 seen, these islands in remote ages probably formed the 

 extreme south-western termination of what was then 

 continental Europe. They formed a part of a great 

 Range Base of species during the Glacial Epoch, of 

 which many traces still endure, either in the form of 

 endemic races or species, or in the annual passage to 

 and from them of many migratory birds. Their situa- 

 tion, however, at the very south-western limits or ex- 

 tremity of a refuge area, or range base on the northern 

 limits of a wide expanse of water across which we 

 have no evidence that purely terrestrial species pene- 

 trated, is a most favourable one for the establishment 

 of endemic forms. The Canary Islands still continue 

 to be the resort of a great many northern species in 

 winter, but there is no evidence of any strong migration 

 across them from north to south, for reasons which 

 have already been dwelt upon. This group of islands 

 within the past few years has received considerable 

 attention from ornithologists, among whom may be 

 mentioned Canon Tristram, Mr. Meade Waldo, and Herr 

 Konig. The result of these investigations has been to 

 show that the Canary Islands arc, comparatively speak- 



