PARTULA. 163 



^valleys. The topographic harriers (ridges unsuitable for 

 Partulae) are evidently older than the races so isolated, which 

 are the modified descendants of stocks once widely diffused. 

 Diverse mutations soon result in racial differentiation in 

 -such isolated colonies, whether the process can be aided by di- 

 versity of environment or not. On a small scale this is illus- 

 trated by the 'Colonies of Helix nemoralis in Burlington, New 

 Jersey, all descended from a single colony planted about 50 

 years ago, yet now showing incipient racial traits in the 

 several colonies scattered over the town. 



Gulick, in his ''Evolution, Racial and Habitudinal" p. 220 

 has directed attention to the fact that arboreal snails do not 

 have the facility in migrating enjoyed by terrestrial species, 

 and hence are more subject to local differentiation brought 

 about by isolation. 



Dr. A. G. Mayer writes : "It is probable that geographical 

 isolation plays a most important part in the formation of 

 new species. If two valleys be adjacent, their snails are 

 closely related each to each, whereas the wider the separation 

 between any two valleys, the more distant the relationship 

 between their snails. The ridges between the valleys, being 

 either barren or covered with vegetation unsuitable to the 

 snails, affords barriers over which the animals must find it 

 more or less difficult -to pass. Thus the Partulae in the 

 Tahitian valleys are isolated very much as are the Achati- 

 nellidse of Oahu in the Hawaiian Islands. 



"In Tahiti the snails are most abundant in the valley- 

 bottoms, where they usually occur on the under sides of the 

 leaves of Caladium and Plantain, although in some valleys 

 they are frequently found on DracaBna and Turmeric. Al- 

 though more abundant in the bottom, they extend for some 

 distance up the sides of the valley and appear to be present 

 in most places where the plants which they affect are found. 

 As far as the very limited observation of the writer goes, 

 there appears to be no difference in the character of the 

 snails in different parts of the same valley. The difference 

 between any two adjacent valleys is, however, very marked." 



