GEOGRAPHIC DISTRIBUTION OF ANIMALS: OCEANIC ISLANDS 293 



the Cape de Verd Islands bearing the impress of Africa, as the inhabitants 

 of the Galapagos Archipehigo are stamped with that of America." Such 

 relationships of island faunas to those of neighboring continents are to be 

 expected if evolution is a fact but are inexplicable upon any other basis. 



In the second place, what is the significance of the fact that some Gala- 

 pagos birds resemble American species more closely than do others? Lack 

 is doubtless correct in explaining this fact upon a basis of the differing 

 lengths of time during which the different species have been inhabiting the 

 archipelago. Thus the cuckoo is probably a relative newcomer, so recently 

 arrived that it has not had time to develop differences from its South 

 American ancestors. The mockingbird, on the other hand, arrived much 

 earlier, a fact evidenced by its greater degree of difference from mainland 

 mockingbirds as well as by its differentiation into separate species and races 

 on the various islands. Undoubtedly animals differ in rate of evolutionary 

 change; conclusions correlating degree of difference with length of time 

 during which isolation has been operative are probably valid in the main, 

 however. (See discussion of faunal stratification, p. 261.) 



Following this line of thought we conclude that the ancestors of Darwin's 

 finches were very early migrants to the archipelago, perhaps the first birds 

 to reach it. These finches differ greatly from any other living finch and have 

 developed many island forms. 



Finches belong to the largest family of birds, the Fringillidae, which in- 

 cludes many of our most common birds — among them our captive songster, 

 the canary, the many species of sparrows, the goldfinch, the grosbeaks, 

 and the cardinal. In the words of Chapman (1920), birds of this family 

 "generally agree in possessing stout, conical bills, which are admirably 

 adapted to crush seeds." With this fact in mind we direct our attention to 

 Darwin's finches, characterized by Lack as follows: "Darwin's finches are 

 dull to look at, not only in their orderly ranks in museum trays, but also 

 when they hop about the ground or perch in the trees of the Galapagos, 

 making dull unmusical noises. Only the variety of their beaks and the num- 

 ber of their species excite attention — small finch-like beaks, huge finch-like 

 beaks, parrot-like beaks, straight wood-boring beaks, decurved flower- 

 probing beaks, slender warbler-like beaks; species which look very different 

 and species which look closely similar." 



How did such diversity arise? When the finches first reached the islands 

 they found many "unfinchlike ecological niches" open to them. Probably at 

 first they had few, if any, enemies, though there are now two species of 

 owls which prey upon them. Under such conditions the numbers of finches 

 would have been limited only by the available food supply. Since the quan- 



