1920.] Birds of the Canary Islands. 553 



the Canariaii form has in a previous epocli crossed from a 

 home among: the sandy wastes of Africa, and finding itself 

 in an island (Fuerteventnra) geologically different from tlie 

 Saharan deserts has become modified in just such a way 

 as best to afford it protection from its enemies by the 

 remarkable resemblance of its plumage to the ground on 

 which it lives; for the back of the island bird has become 

 darker than that of the Continental form, therebv being less 

 conspicuous among the lava rocks, with which the Fuerte- 

 venturan plains are scattered, than Avould otherwise have 

 been the case. Tlie Fuerteventuran Bustard is now regarded 

 as a perfectly distinct geographical race and has been named 

 Chlamydofis undulata fuerteventnra. 



In this connexion it is interesting to note that in the 

 eastern Canary Islands, where the reefs which are found on 

 many parts of the coast are formed of black water-worn 

 lava, the Oystercatcher is black in plumage^ and were it not 

 for its red bill — which must serve some purpose to aid it in 

 its search for food — would be most inconspicuous. 



Variation, then, is commenced in a species by any one of 

 the factors which we have been discussing, or more probably 

 by a combination of most or all of them. It is obviously 

 impossible to say which has had the greatest influence 

 on the island birds that are the subject of this paper. 

 JNIany of them are only very slightly modified, as, for 

 instance, the Trumpeter Bullfinch, Least Goldfinch, Madeiran 

 Rock-Si)cirrow, Koenig's Shrike, Spectacled Warbler, etc., etc. 

 The variation having once started it is maintained by 

 natural selection — that process whereby certain of the same 

 species are more or less rapidly eliminated while others are 

 able to survive and thrive. This is the natural outcome of 

 the struggle for existence ; only a certain number of birds 

 can exist in an island such as Hierro or Lanzarote, if thev 

 continually keep on increasing : finally, there must be a 

 scarcity of food — particularly in desert islands, — the birds 

 fight for the food amongst themselves, the fittest survive, the 

 weakest perish. Thus the progressive modification of species 

 by the agency of variation and natural seleeiion which we 



