SOURCES OF CHANGE 69 



the original inhabitants of a region may bring in 

 certain hereditary characters which distinguish 

 them from members of the same species in other 

 regions. As they reproduce, their descendants ex- 

 press an aggregate of the predominant characters, 

 and thus perpetuate the difference. Such differ- 

 ences may characterize geographic races or, by the 

 vagaries of taxonomic procedure, subspecies or 

 varieties. They need not be adaptive but may be 

 purely incidental, and they are due solely to the 

 isolation of a given hereditary combination. 



The sharpness of boundaries in such cases must, 

 of course, go far to establish the definiteness of the 

 subspecific units. Oceanic islands like the Gala- 

 pagos archipelago provide an absolute isolation of 

 long duration and so, quite naturally, afford the 

 best illustrations. The mocking birds of the Gala- 

 pagos are of several closely related species, found 

 on separate islands, their distinctive characters 

 apparently without adaptive significance.^^ The 

 ground finches of the same islands show a remark- 

 able series of species or forms based partly on the 

 size of the beak, which varies from enormously 

 large to rather small without correlation with food 

 habits, but in this case the various species are not 

 sharply isolated. Wheeler notes of two species of 

 Galapagos ants that "each of them is represented 



'6 Giflford, E. W., Proc. Cd. Acad. Sci. (4), II (2), p. 207, 1919; Beebe. 

 William, Galapagos, World's End, 1924. 



