216 APPENDIX ii INTENSIVE; SEGREGATION. 



ophy. Darwin is more guarded in his statements ; still, as we have 

 already shown, he sometimes seems to reason from an assumption quite 

 in accord with what Spencer would have us receive as essential to the 

 very idea of causation in vital processes. For example, his explanation 

 of the fact that on the different islands of the Galapagos Archipelago 

 one genus is, in many cases, represented by several closely allied 

 species which are undoubtedly modified forms of one continental 

 species, seems to rest on the assumption that if every species that 

 gained access to any island had at the same time gained access to the 

 other islands of the archipelago, there would then have been no occa- 

 sion or opportunity for the divergences we now find. (See Origin of 

 Species, 6th ed., p. 355.) 



It seems to me that the divergences presented by the varieties and 

 species of the family Achatinellidas are at variance with this assump- 

 tion. Not only are islands in sight of each other occupied by diver- 

 gent species, but different parts of the same mountain range exposed 

 to the same winds and rains and clothed with the same vegetation are 

 the homes of divergent forms. 



Turning to the map of the island of Oahu, we find a mountain range 

 extending 36 miles from northwest to southeast, nearly parallel with 

 the northeast coast. The northeast side of this range is exposed to 

 the trade-winds fresh from the ocean, and accordingly receives a 

 heavier rainfall than the other side ; but there is not much difference 

 in the amount of rain received by the different valleys on one side of 

 the mountain. In nearly all these valleys on either side of the range 

 are found shady groves of what the natives call the "kukui" (Aleu- 

 rites triloba). Many species of the genera Achatinella and Bulimella 

 have their haunts in these groves, some species clinging to the leaves 

 and young branches, and others to the trunks and the larger branches. 

 Most of the species thrive only where the shade is dense and the atmos- 

 phere laden with dampness a large portion of each month. 



The student who starts with the assumption that divergent varie- 

 ties and species arise only through exposure to different environments 

 will expect that these groves, at least those on the same side of the 

 mountain range, will be occupied by the same species. Having found 

 one set of species in a given valley, when he comes to a valley ten 

 miles distant, possessing the same conditions of soil, rainfall, vegeta- 

 tion, and shade, where the birds, reptiles, and insects are the same, 

 where the mice and ants, their only known enemies,* are the same, he 

 naturallv looks on the leaves and branches of the familiar trees for 



* The species that molest the snails were not known on these islands till com- 

 paratively recent times. 



