GEOGRAPHIC ISOLATION AND SPECIES-FORMING 127 



closely related Psittacirostra eats only fruits, that of the ieie 

 (Freycinetia arbor ea) and the red mulberry (Morns sapyrifera) 

 being especially chosen. In all these genera, there is prac- 

 tically one species to each island, except that in some cases 

 the species has not spread from the mountain or island in which 

 we may suppose it to have been originally developed. 



There are a few other song birds in the Hawaiian Islands, 

 not related to the Drepanida3. These are derived from the 

 islands of Polynesia and have deviated from the original types 

 in a degree corresponding to their isolation. In the case of the 

 Drepanidae, it seems necessary to conclude that natural selection 

 is responsible for the physiological adaptations characteristic 

 of the different genera. Such changes may be relatively rapid, 

 and for the same reason they count for little from the stand- 

 point of phylogeny. On the other hand, the nonuseful traits, 

 the petty traits of form and coloration which distinguish a 

 species in Oahu from its homologue in Kauai or Hawaii, are 

 results of isolation. These results may be analyzed as in part 

 differences in selection with different competition, different 

 food and different conditions, and in part to hereditary differ- 

 ence due to the personal eccentricities in the parent stock from 

 which the newer species was derived. 



In these as in all similar cases we may confidently affirm: 

 the adaptive characters a species may present are due to 

 natural selection or are developed in connection with the 

 demands of competition. The characters nonadaptive which 

 chiefly distinguish species do not result from natural selection, 

 but are connected with some form of geographical isolation 

 and the segregation of individuals resulting from it. 



The origin of races and breeds of domestic animals is in 

 general of much the same nature. In traveling over Eng- 

 hiiul one is struck by the fact that each county has its own 

 breed of sheep, each of these having its type of excellence in 

 mutton, wool, hardiness, or fertility, but the breeds distin- 

 guished by characters having no utility either to sheep or to 

 man. The breeds are formed primarily by isolation. The 

 traits of the first individuals in each region are intensified by 

 the inbreeding resulting from segregation. Natural selection 

 preserves the hardiest, the most docile, and the most fertile: 

 artificial selection those which yield the most wool, the best 

 mutton and the like. The breed once established, artificial 



