INTENSIVE SEGREGATION. 355 



I know of but one species that seems to be distributed over the 

 vvliole 36 miles of the main mountain-range, and this one is 

 represented by tiiree varieties belonging to different parts of the 

 range and perhaps worthy to be regarded as different species. 

 The species to which I refer is Auriculella auricula (Fer.), the 

 typical forms of wliich are found on the eastern half of the 

 mountain-range. On the other half of the range we find the 

 closely allied forms to whicK I have given the manuscript names 

 sollda and pelluc Ida. This great contrast in the size of the areas 

 occupied must be due either to the greater plasticity of the 

 Sandwich-Island species, or to their having inferior opportunities 

 for migrating, or to both causes. As I become better acquainted 

 with the great difference in the habits and circumstances of the 

 contrasted species, I give increasing weight to the difference in 

 the opportunities for migrating. With the continental species, 

 floods must be one great means of distribution ; but in the case of 

 the insular species, the floods would carry floating individuals upon 

 the grass-land or into the sea, in either case to perish. Again, 

 the habit of travelling upon the ground, which belongs to most 

 of the Helices of Europe and America, gives incalculable oppor- 

 tunities for migration which are not enjoyed by species that are 

 strictly arboreal, as are many of the Sandwich-Island species. 

 Most of the Sandwich-Island species are still further restricted 

 in their opportunities by their inability to resist a dry atmosphere 

 or exposure to the sun, which renders it necessary that they 

 should remain in the isolated areas that are fa\oured with shade 

 in the different valleys. 



The habits of the different subgenera occupying Oahu are also 

 instructive as throwing light upon the relative areas occupied by 

 the species of the different genera. AcJiatinella and Bulimella 

 seem to be the most restricted in their opportunities for migrating : 

 first, because they are entirely arboreal in their habits, clinging 

 to the trunks and branches of trees through their whole life- 

 history ; and, second, because, for the most part, they occupy 

 the shady and damp thickets and groves, the shade in each valley 

 •being separated from similar shades in adjoining valleys by lofty 

 and sparsely wooded mountain-ridges at each side of the valley 

 and by open grass-land at the mouth of the valley. On the other 

 hand, Jjt?e^, which for the most part occupies trees and shrubs on 

 the ridges which are connected with each other through the 

 central ridge of the mountain-range, and Amastra and Lepta- 



