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 Achatinellidae 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. I am 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. I am 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 Zoology at Harvard University, 

 vol. LX, No. 5. 



