INTRODUCTION. Il 
tule and of other terrestrial mollusca. Secondly, it was deemed desirable to carry 
further the work begun by Mayer upon the inheritance of shell and color charac- 
teristics, which are so important for the determination of the continuity or dis- 
continuity of related forms. The material collected for the first purpose fortunately 
serves the second as well, for the snails are viviparous and many of the adult char- 
acters under investigation may be determined with complete or substantial accuracy 
for the young carried in the brood-pouch for weeks after development is begun, 
contrary to the opinions of Hartmann and others who have not been familiar with 
freshly caught material. In the third place it was planned to bring back living 
specimens to see if they would serve for laboratory experiments upon the course of 
inheritance in pure and hybrid strains; although many hundreds of snails have 
been successfully transported from the islands to the laboratory, and have been 
maintained alive for several months, it long since became evident that the work in 
the other departments must take precedence and must be completed before satis- 
factory experiments upon heredity can be prosecuted with entire advantage. 
During the four journeys, more than 80,000 adult and adolescent individuals 
were obtained from over 200 valleys of larger and smaller size, in the Society Islands 
alone; the additional number of young snails contained in the brood-chambers of 
the adult specimens has not yet been fully determined, but the general average of 
embryonic snails that are sufficiently developed to display definite characters has 
proved to be about two for each adult of the several thousand already studied; thus 
there is ample material for the solution of certain perplexing problems dealing with 
the heredity of the diagnostic peculiarities of species and varieties. 
In view of the length of time necessary for the thorough analytical study of 
the wealth of material already collected, it seems inadvisable to defer the publication 
of results until the entire research may be completed. The present memoir deals 
with those species only which occur in Tahiti, the largest of the Society Islands, 
although it includes a discussion of the features of the Polynesian area as a whole. 
In the course of the whole investigation, certain definite conclusions of a general 
nature have been confirmed or newly established, and for the sake of perspective it 
is desirable to give them in preliminary outline at this juncture: 
(1) The snails are far from uniform in their distribution; with only one excep- 
tion each group of islands has its own characteristic species which occur nowhere else. 
(2) The same correlation between geographical and specific discontinuity is 
displayed by the species of the different islands of one and the same group, for each 
member possesses distinct species not found in the others, although in a very few 
instances important exceptions occur which throw much light upon the processes 
of dispersal and migration as well as upon certain geological relations. 
(3) The species of Partula, like the Achatinellide of the Hawaiian Islands, may 
vary from valley to valley of an island; one form sometimes extends over a wide 
range, while another may be restricted to a few valleys or even to one; less obvious 
differences are displayed by types which inhabit different parts of a single valley. 
Statistical results prove the essential difference of races belonging to a species that 
has been heretofore regarded as invariable, while in other cases close relationships 
