220 APPENDIX ii INTENSIVE; SEGREGATION. 



(5) The average radius of distribution for species of the same value in 

 allied genera varies in the different genera directly as the average power 

 and opportunity for migrating, and inversely as the plasticity and vari- 

 ability of each genus. Comparing the distribution of the Helices of 

 Europe with that of the Achatinellidae of Oahu, the most striking con- 

 trast is found in the size of the areas occupied by the different species. 

 Helix pomatia is distributed from England to Turkey, over an area 

 2,000 miles in length, while of the eight genera of Achatinellidae on 

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

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

 resented by three varieties belonging to different parts of the range 

 and perhaps worthy to be regarded as different species. The species 

 to which I refer is A uriculella auricula (Per.) , the typical forms of which 

 are found on the eastern half of the mountain range. On the other 

 half of the range we find the closely allied forms to which I have given 

 the manuscript names solida and pellucida. This great contrast in the 

 size of the areas occupied must be due either to the greater plasticity 

 of the Achatinellidae 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 grassland or into the sea, 

 in either case to perish. Again, the habit of traveling upon the ground, 

 which belongs to most of the Helices of Europe and America, gives 

 incalculable opportunities for migration which are not enjoyed by 

 species that are strictly arboreal, as are many of the Hawaiian species. 

 Most of the Hawaiian snails are still further restricted in their oppor- 

 tunities 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 favored with shade in the different valleys. 



The habits of the different genera occupying Oahu are also instruc- 

 tive as throwing light upon the relative areas occupied by the species 

 of the different genera. Achatinella 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 adjoin- 

 ing valleys by lofty and sparsely wooded mountain ridges at each side 

 of the valley and by open grassland at the mouth of the valley. On 



