434 EVOLUTION, GENETICS, AND EUGENICS 



or are developed in connection with the demands of competition. 

 The characters, non-adaptive, which chiefly distinguish species do not 

 result from natural selection, but from some form of geographical 

 isolation and the segregation of individuals resulting from it." 



J. T. Gulick, another exponent of the efficacy of geographic isola- 

 tion in species-forming, has offered in evidence of his views facts about 

 the distribution of Hawaiian land snails. In the island of Oahu, for 

 example, the volcanic ridges have been eroded out into a series of iso- 

 lated valleys in the bottoms of which grows abundant vegetation, 

 while on the highlands there is little but barren rock. The climatic 

 conditions of all the numerous valleys are the same, but, remarkably 

 enough, each variety of snail is confined not only to one island, but to 

 a definite valley on an island. The degree of difference, moreover, 

 between varieties is in proportion to the distance that separates them. 

 Gulick claimed that he was able to estimate the degree of divergence 

 between the snails of any two valleys by measuring the number of 

 miles that lay between them. Gulick's findings have been extensively 

 corroborated by recent explorations on the snails of other oceanic 

 islands by Crampton. 



An interesting type of isolation that hardly can be termed geo- 

 graphic, yet is essentially equivalent to the latter in its effects, is found 

 in connection with the extensive group of lice (Mallophaga) that live 

 their whole lives buried among the feathers of birds or the hair of 

 mammals. These animals cannot fly and are quite effectively isolated 

 for life upon a particular bird. They do, however, during the intimate 

 period of nesting, pass from parent to offspring, so that they may be 

 said to be isolated upon definite genetic lines. In the case, especially, 

 of birds like the eagle, a bird of long life and monogamous habits, the 

 parasite becomes as isolated as might be a race on a small island. The 

 result is that sometimes the lice of a single bird and its offspring are 

 of quite a distinct variety, which has become fixed by inbreeding until 

 a high degree of uniformity has been attained. Such an isolated 

 variety may be almost as distinct as a true species. Obviously in this 

 case, as in others, isolation must have had a real effect upon species- 

 forming quite apart from natural selection, except in so far as the unfit 

 variants have not survived. 



Populations of 'lakes and ponds — Just as islands of land in seas 

 of water serve to isolate terrestrial forms, so islands of water in seas 

 of land, if we may be pardoned the expression, just as effectively isolate 

 aquatic organisms. Thus, isolated ponds and lakes commonly harbor 



