1920.] Birds of the Canary Islands. 568 



occurrence of tlie migratory species in these islands is 

 not by any means a matter of chance alone. 



To convince ourselves that the Canary Islands have for 

 centuries been in tlie direct flight of migratory birds we 

 have on]}^ to glance down the list of the Resident species 

 (non-migratory at the present day). Among these we see 

 certain birds which with hardly -Awy doubt got a footing in 

 the islands when the species (not necessarily the geo- 

 graphical race into which some have now evolved) passed 

 on its regular flight through the islands : the Chiff'chaff, 

 Warblers, Kestrels, Woodcock, and ^Nladeiran Quail come to 

 mind as typical examples, most of which have now entirely 

 ceased to migrate, and all of which, save the Woodcock, 

 have become difterentiated to a lesser or greater extent. 



In Dr. Lowe's fascinating book 'A Naturalist on Desert 

 Islands^ (p. 48) the question is asked, "How came these 

 biids to drop their migratory habit ? Did the climate and 

 the conditions generally, in the Canaries, gradually come to 

 fulfil exactly throughout the whole year the requirements 

 of the (Jhiftchaft', and so gradually do away with the neces- 

 sity for periodical rnigration ? Under these conditions we 

 can conceive that those birds which did return annually to 

 Europe in the spring would gradually become fewer and 

 fewer, until at length there would be none left, and this 

 migratory branch-route north to south or south to north 

 would cease to exist, and the Canary Island birds wonld be 

 cut off from any autumnal influx of birds which had bred 

 in the north, and would be completely isolated." In a later 

 chapter Dr. Lowe remarks : "Isolation, of itself alone, does 

 not seem capable of producing fresh varieties any more than 

 segregation. Natural selection is only the final arbiter in 

 determining what variations shall survive, after they have 

 been produced by the influence of external conditions, Jf 

 the external conditions are the same all the world over, 

 natural selection cannot come into action," 



The same thing is going on to-day : witness those species 

 which I consider Partial Residents and which have not vet 

 entirely dispensed with the migratory habit; such are the 



