224 APPENDIX II—INTENSIVE SEGREGATION. 
But why should the degree of divergence increase with the contin- 
uance of the isolation? The answer seems to be that the combined 
effects of the different principles of transformation in the isolated 
groups increase with the time of isolation; and, as independent 
transformation is never parallel, the divergence increases in the same 
ratio. ‘Diversity of environal selection is undoubtedly one of the prin- 
ciples producing this divergence, even when the vegetation and 
physical conditions of the different districts are the same, for when the 
habits of feeding change, the environal selection must usually change. 
But there are cases of divergence accompanying isolation in which 
the habits of feeding seem to have remained unchanged; and in such 
cases I explain the divergence by the fact that any small fragment of 
a species is incapable of propagating all the qualities of the species in 
the exact proportion presented by the average of the species. 
2. Similar Facts in Other Regions. 
Many of the facts embodied in these eight propositions must have 
been observed wherever naturalists have studied the geographical dis- 
tribution of the varieties and species of polymorphic genera; but in the 
distribution of the Achatinellide there are features of peculiar interest 
arising from the fact that the powers of migration possessed by the 
species of the surrounding environment are very much greater than 
those possessed by these molluscan species. Through this circum- 
stance a comparatively uniform environment is produced in which the 
effects of independent generation unmodified by the effects of changed 
environment may be observed. The remarkable facts of distribution 
which we have on the island of Oahu are found in other parts of the 
Hawaiian Islands, wherever this family occurs. Iam also fully con- 
vinced that, in other parts of the world, wherever one genus or family 
of very low powers of migration is surrounded by a body of plant and 
animal forms possessing much higher powers of migration, there similar 
facts will present themselves whenever investigation is made. 
The distribution of land mollusks belonging to the genus Partula 
found on the Society Islands present similar features. The island of 
Reiatea, which is but 14 miles in length and 3 or 4 miles in breadth, is 
the home of about 30 species and varieties, most of which are confined 
to areas only a few square miles in extent. Jam not informed as to 
the distribution of the plants on which these species feed, but there is 
no reason to suppose they occupy limited districts corresponding to 
those occupied by the different species of Partula.* 

* See Bulletin of the Museum of Comparative Zodlogy at Harvard University, 
vol. Lx, No. 5. 
