PARTULA NODOSA. 107 
the original smaller area of occupation seems to have been a portion of Punaruu 
Valley. 
But we could not justify the hypothesis that P. nodosa arose de novo, so to speak, 
either in Punaruu or in any other valley; this species must have had antecedents 
from which it has come to differ more or less radically. No other species of its 
islands displays characters which might be deemed ancestral to those of nodosa. 
P. hyalina and P. clara have attenuated and elongated shells, together with other 
distinct differentia; P. otaheitana varies greatly in Vahiti, but its members, even in 
valleys situated in or adjacent to the territory of nodosa, are readily distinguishable. 
Only P. filosa resembles nodosa in form to an appreciable degree, a fact which is 
undoubtedly significant; but it is sharply restricted to a single valley in the dry and 
distant northern sector of Tahiti nui, from which migrants could not make their 
way to the western valleys, there to develop the distinctive qualities of nodosa as 
such. But outside of Tahiti occurs one species that does resemble nodosa in colora- 
tion as well as form, namely, P. suturalis or P. lineata of southern and western 
Moorea. ‘The discussion narrows down, then, to these three species, nodosa, filosa, 
and suturalis, and two hypotheses as to their inter-relationships present themselves. 
The first is that P. nodosa is derived from individuals of the Moorean P. 
suturalis, introduced into the western sector of Tahiti by human agency. Once 
established, migrants to other localities would extend the range of the species, as we 
have shown. In this case, P. filosa would scarcely be a relative of nodosa and a 
product of this newly introduced stock, for it would be just as difficult for snails to 
cross the island from Punaruu to Pirai Valley as to accomplish the reverse journey; 
and, furthermore, nodosa has not been able to populate more than a small territory 
outside of its original headquarters. Such a view as to the relationship between 
nodosa and suturalis might simplify the problem of the former’s origin, but it would 
still leave the ancestry of the latter to be determined; it would be conceivable also 
that the opposite might be the actual case, namely, that nodosa is ancestral to 
suturalis by human means of introduction from ‘Tahiti to Moorea, which, if true, 
would still leave the Tahitian problem unsolved. Finally, human intervention has 
not been demonstrated as a constant or even as a casual factor in the dissemination 
of species of Partula. It is certainly not more likely that nodosa would be humanly 
introduced into Moorea, or suturalis into Tahiti, when each species is absent from 
valleys of its own island located not far distant from the colonies actually existing, 
whose lower parts are continually visited back and forth by natives. 
By an alternative hypothesis, the facts may be interpreted more reasonably. 
According to this, P. nodosa is descended from an ancient widespread stock, differing 
from the progenitors of other Tahitian and Moorean species that existed through- 
out the land-mass which included the now separate islands in question. This stock 
differentiated into the more immediate ancestors of nodosa in Tahiti and suturalis 
in Moorea. At first the nodosa series extended widely around Tahiti, but subse- 
quently the colonies living in most of the valleys lost their vigor and disappeared, 
as certain colonies of P. hyalina and of other species seem to have done more 
recently. P. filosa is a local relic of the same original stock, if we regard its resem- 
