1920.] Birds of the Canary Islands. 549 



their relations were not nmcli disturbed, and that if 

 variation took place this would have been all iu the same 

 > direction. 



The fact that neither the Courser of the Canaries nor the 

 Sand-Grouse has become modified in any way may be 

 attributed to the almost identical conditions which exist 

 between the continental and island homes of these birds, 

 although we might reasonably have expected the island 

 forms to have become darker, as the Bustard has done. 

 Possibly migration to the mainland takes place, but we have 

 no proof of this in either case. 



The fact that the Ospiey is similar to the continental 

 form is easily understood, for it can hardly be termed isolated 

 in the Archipelago^ and being a coast-frequenting species 

 would not meet with im environment much altered from 

 continental conditions. 



Similarly, none of the birds which I have termed Partial 

 Residents in the Canary Islands have become differentiated, 

 th.e frequent arrival of unmodified migrants from the main- 

 land wiih which they interbreed being sufficient to keep up 

 the continental strain. The Hoopoes, however, are making 

 a great fight to establish themselves as an island race. 



Isolation is one of the strongest factors — if not the 

 strongest factor — to be reckoned with in the (lifi'ei'entiation 

 of a species. Once a bird is isolated and cut off from the 

 stock from which it sprang, whether of continental or insular 

 origin, it can no longer keep up the old strain ; and unless 

 all the new conditions are exactly similar to those which it 

 has left, the bird begins to develop along its own lines, 

 influenced by the changed environment in which it finds 

 itself. 



In the Canary Islands nujst of the birds are com[)letely 

 isolated and many are confined to this Archipelago, where 

 perfectly distinct geographical races have been ibrmed ; 

 but in one or two cases where insular races have been 

 recognised, this isolation is not complete. Tiie llt'sident 

 Canarian Qnail (^Cutwiii.r c. confisa) occasionally interbreeds 

 with the Migratory Quail, which visits and biecds in the 



