86 VARIATION, DISTRIBUTION, AND EVOLUTION OF THE GENUS PARTULA. 
removed in the lighter or darker direction. The collection from the lower valley, 
taken in 1906, is not essentially different from the series of the upper valley, obtained 
in 1909, as regards the relative numbers representing the several color divisions. 
SUMMARY AND CONCLUSION. 
The noteworthy feature of distribution is the rigid confinement of filosa to a 
single valley of Tahiti, Pirai. Notwithstanding this restriction, the shells of this 
species exhibit a remarkable degree of variation in color and in form. 
The dryness of the northwestern section, in which Pirai Valley lies, accounts 
for the inability of the Pirai colony to send migrants into adjacent and equally 
suitable regions. 
We may view this single valley association of filosa as the sole relic of a form or 
series of forms that in earlier times possessed a far wider range, but whose repre- 
sentatives in other localities of Tahiti have disappeared on account of their constitu- 
tional inability to maintain themselves; the only other form of Tahiti that is allied 
to filosa is P. nodosa—a somewhat distant connection. In other islands of the - 
group, relatives of filosa are to be found that have evolved from the same ancestral 
stock along other lines of modification. 
