470 ME. O. SALVIN ON THE AVIFAUNA 



gable Island three, Bindloe three, and Abingdon four. Thus we see that in Chatham 

 Island every gradation in size is represented. 



Then as to coloration. The assumption of the black plumage by the old males seems 

 to be a slow process, the fully black feathering being only arrived at in probably the 

 third year; and it is probable, from the comparatively scanty numbers of the black 

 individuals, that the cock birds breed in the mottled plumage long before the fully 

 black dress is assumed. Observations on paired and nesting birds would here be 

 invaluable. 



It would seem, then, that with these singular birds the sexual selection displayed 

 amongst them is such that it is almost a matter of indifference whether the cock birds 

 are mottled or black, and also that the nature of their food and the general conditions 

 of life are such that birds with huge bills as large as a Grosbeak's, as well as those 

 with bills no bigger than a Bunting's, can equally find sustenance, variation as regards 

 the length of wing, tail, and tarsi being equally unimportant. The members of this 

 genus present a field where natural selection has acted with far less rigidity than is 

 usually observable. 



The gap between Geospiza and Cactornis is fairly defined — not but Mhat we see in 

 some specimens of the former more elongated bills than in others, showing a tendency 

 in Geospiza to develop in the direction of Cactornis. But it would seem that the con- 

 necting links are gone; hence our ability to define the difi'erences between these 

 genera. 



Camarhynchus belongs to a somewhat difierent type ; and I am inclined to believe 

 that it is not to be traced to the same origin as Geospiza and Cactornis, but to a 

 common ancestor with, perhaps, Spermophila, the bill of the somewhat abnormal 

 ally of that genus {Neorhynclms nasesus) recalling to some extent the peculiarities of 

 the bill of Camarhynchus. So much for structural characters. In coloration Cama- 

 rhynchus resembles both Geospiza and Cactornis ; but the males of none of the species 

 are so black as those of the adults of the other genera. 



It seems worthy of notice that, though the different species of Finches are not 

 restricted to any one particular island, there appears to be a prevalence of some one 

 species in each. Dr. Habel's collection shows that in Indefatigable Island two dominant 

 species occur — Geospiza fortis and G. fuliginosa. In Bindloe Island G. strenua abounds, 

 whilst in Abingdon we find G. parvula. It may thus be argued that the tendency of 

 each of these islands is to produce the form most prevalent in it ; but, on the other 

 hand, I am not able to say what attention Dr. Habel devoted to each species. 



