PARTULA. 131 



These color-sports tend to breed true to tliein-jelves, :ind therefore to oi'igi- 

 nate new color-forms and finally new species. This tendency is, however, 

 held in check by frequent inter-crossing with the parent stock, and becomes 

 effective only when the new color-variety is isolated, or when it displays 

 a remarkably strong tendency to breed true. A good example of a color- 

 variety that tends to breed true, but which is nevertheless held in check 

 by frequent inter-crossing with the parent stock, is afforded by the type 

 of snail represented in Fig. 6, which is derived from the type shown in 

 Fig. 5, both coming from Tipoerui valley. Here we find that 59 per 

 cent of the young of Fig. 6 resemble the normal young of Fig. 5, while 

 8 per cent of the young of Fig. 5 exhibit the whorl-stripe character- 

 istic of the young of Fig. 6. 



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 vege- 

 tation unsuitable to the snails, afford barriers over which the animals must 

 find it more or less difficult to pass. Thus the Partuloe in the Tahitian 

 valleys are isolated very much as are the Acliatinellidae 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 Dracajna and Tur- 

 meric. Although 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 tlie 

 character of the snails in different parts of the .same valley. The difference 

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



All of the snails of Tipajrui valley are dextral, while all of the same 

 species in Pirte valley are sinistral. In the two intermediate valleys of 

 Hamuta and Fautaua some individuals aie dextral and some sinistral. 



