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 Achatinellide 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 Achatinellide on 
Oahu I know of but one species that seems to be distributed over 
the whole 36 miles of the main mountain range, and thisoneis 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 (Fer.), 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 Achatinellide 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 
