OTHER THEORIES OF SPECIES-FORMING 271 



effects, but remaining uniform in most regards, all representatives 

 slowly changing together in the process of adaptation by natural 

 selection. In regions broken by barriers which isolate groups of indi- 

 viduals we find a great number of related species, though in most cases 

 the same region contains a smaller number of genera or families. In 

 other words, the new species will be formed conditioned on isolation, 

 though these same barriers may shut out altogether forms of life which 

 would invade the open district. 



" Given any species in any region, the nearest related species is not 

 likely to be found in the same region nor in a remote region, but in a 

 neighboring district separated from the first by a barrier of some sort. 



"Doubtless wide fluctuations or mutations in every species are 

 more common than we suppose. With free access to the mass of 

 the species, these are lost through interbreeding. Isolate them as in 

 a garden or an enclosure or on an island, and these may be con- 

 tinued and intensified to form new species or races. Any horticul- 

 turist will illustrate this. 



"In all these and in 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, 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 

 isolated 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 Cramp ton. 



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

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



