Escpedition to the Eastern Canary Islands. 229 



gracilirostris) and the Fuerteventurau Bustard {Chlamydoti 

 undulata fuerteventuras)^ and compare their claims to sub- 

 specific rank with those of the Sardinian Warbler {Sylvia 

 melanocephala leucogastra) and the Canary Island Shrike 

 (Lanius excubifor Jcoenigi). It is obvious that the first two 

 mentioned have become much further removed from the 

 parent form than the last two, in which very slight modi- 

 fication has taken place. Whether the Bustard and the 

 Slender-billed Barn Owl have become more highly charac- 

 terised in consequence of longer residence in the Canary 

 Islands than the Shrike and Warbler, it is impossible to say 

 for certain. For we have no means of ascertaining the exact 

 period when any of these birds first became inhabitants of the 

 group. In considering the elements which constitute such 

 an important part in the forming of a well-defined insular 

 race, we have, in the Canary Archipelago, to consider the 

 geographical position of the group in relation to Africa, the 

 climatic conditions of the various islands, their altitude, and 

 particularly the physical environment with which the birds 

 are surrounded. As time is the main cause of any species 

 which is isolated from the typical form becoming modified, 

 it might be supposed, and possibly in some cases correctly, 

 that the first birds which arrived in. the Canary Islands are 

 those which show the greatest difi'erentiation at the present 

 day, e.g., that the Slender-billed Barn Owl and the Fuerte- 

 venturan Bustard became isolated in the Canary Islands many 

 j^ears in advance of the Sardinian Warbler and the Canary 

 Island Shrike. In considering this problem we must bear in 

 mind that the several elements mentioned above undoubtedly 

 act very diflPerently on the various species which now inhabit 

 the group. Another important factor to be reckoned with 

 is the conditions under which the bird lived prior to its 

 having gained a foothold in one or more of the islands. If 

 the conditions are much the same we shall probably find that 

 a very slight change, if any, has taken place in the bird. For 

 instance, the Cream-coloui"ed Courser (Cursorius g. gallicus^ 

 and the Sandgrouse [Pterocles arenarius) which inhabit the 

 islands of Fuerteventvra and Lanzarote are obviously immi- 

 grants from the neighbouring African coast, and finding, iu 



