646 Erste Sitzung der sechsten Sektion. 



similarity of organic forms is correlated with the degree of geo- 

 graphic contiguity. As the various species of this genus are no 

 doubt relatives it is justifiable to conclude that they are the des- 

 cendants of a more uniform ancestral stock belonging to an ancient 

 Pacific continent, which continent through subsidence of the 

 ocean's floor has become divided up into the land masses of the 

 now isolated groups of islands. 



B. When a comparison is made of the species occurring in 

 the different islands of one and the same group it appears that 

 in general eacji particular island possesses peculiar species; again 

 there is a single exception in the case of P. attenuata which is 

 found in both Tahiti and Raiatea. Aside from this exception 

 not a single form of Tahiti is to be found even in Moorea which 

 is only twelve miles from the former. This latter island has its 

 own characteristic snail fauna. In the case of Raiatea and Tahaa, 

 which are four miles apart but encircled by the same continuous 

 coral reef, one species occurs in both islands but the remaining 

 twenty in Raiatea are peculiar and the remaining four of the 

 latter island are peculiar to it. The two halves of Huaheine are 

 still closer together and in correspondence with this degree of 

 nearness one finds close similarities among the indigenous snails, 

 although different varieties occur in the two separated land masses. 

 Therefore the principle established on the basis of a comparison 

 of the species occurring in the different island groups is found 

 to hold true for the species living in the different islands of the 

 same group in so far as biological similarities and geographical 

 contiguity appear to correspond. 



C. Further evidences of the uniformity of this principle are 

 provided by the results of comparing snails occurring in neigh- 

 boring Valleys of one and the same island. For example, in 

 Tahiti, which is ideal as regards the regularity and radial trend 

 of its numerous larger and smaller Valleys, Partula otaheitana is 

 found everywhere, but in Valleys wide apart varieties occur which 

 are more diverse than those of contiguous Valleys. Sometimes a 

 species will occur in a single valley only, as in the case of P. filosa. 

 Again P. clara occurs in perhaps twenty-five Valleys of the total 

 series of one hundred but in different parts of this whole ränge 

 it has differentiated into distinct subordinate varieties. The con- 

 clusion based upon the comparison of valley population agrees 

 consistently with that formulated with reference to the islands 

 constituting a group and to island groups belonging to the Poly- 

 nesian region. 



Additional results. The earlier explorations and 

 studies of Garrett and Mayer with which my own results 

 may be compared makeit possible to demonstrate certain significant 

 changes in the Constitution and distribution of some of the species 

 in Tahiti and other islands. P. nodosa has spread from its original 



