218 BIRDS OF GALAPAGOS. [CH. IV 



had a tendency towards a greyer coloration than the 

 specimens of the same species on the mainland. This alte- 

 ration he was disposed to put down to a colour variation 

 of a protective significance in relation to the abundant grey 

 gneiss of the islands ; it may however be a phenomenon 

 like the other instances that have been just mentioned. 

 It has been suggested that this prevailing melanism of 

 the insects of islands is an effect of the moist climates of 

 islands. On the other hand Mr Wallace noticed a 

 prevalence of white in the colour patterns of some of 

 the butterflies of the Moluccas and New Guinea, a 

 peculiarity more likely perhaps to be due to a possible 

 action of natural selection than the more usual darker 

 colour. The colour in this case would increase the con- 

 spicuousness of the insects ; but in an island they would 

 run less risks from insect-eating foes and might therefore 

 develop more freely in certain directions than on the 

 mainland, where enemies are more numerous and com- 

 petition severer. 



Precisely the same class of characters distinguish the 

 birds of the Galapagos islands 1 . The genus Geospize, a 

 finch peculiar to the islands and consisting of eight species, 

 is of a dull colour to black, the darker hues being the 

 distinctive mark of age. " In old males," says Mr Salvin, 

 " the plumage is almost uniformly black." Such specific 

 names as "fuliginosa" and " nebulosa" keep the pecu- 

 liarities of these insular birds in the mind. Cactornis, 

 another genus restricted to the islands, is "olivaceo- 



1 On the Avifauna of the Galapagos Archipelago. Trans. Zool. Soc. 

 Vol. ix. 1876, p. 447. 



