308 RESUME. 
the continuous area of habitation. Probably the latter points of contrast indicate 
the results of a selective process by which the less vigorous individuals are eliminated 
in unfavorable parts of the valley. This species is the sole relic of a series of forms 
having a wider range in Tahiti in earlier times, but whose representatives have 
disappeared elsewhere on account of their constitutional inability to meet the 
environmental conditions of existence. 
CHAPTER VI. 
The beautiful species described by Pfeiffer as P. nodosa and by Pease as P. 
trilineata inhabits a series of seven consecutive valleys in the western quadrant of 
Tahiti nui, where more than 2,000 full-grown and adolescent individuals were 
collected. The area of occupation is more compact than in the case of P. clara, 
though it is not so contracted as in the case of P. filosa. 
The analysis of the characteristics of the shells, as these are defined both quali- 
tatively and quantitatively, leads to certain significant conclusions regarding the 
past history of the species and its differentiation. ‘The first general point is that 
P. nodosa has expanded its territory of occupation during recent decades. In 
Garrett’s experience it was limited to a narrowly circumscribed territory on one 
side of the stream in the great valley of Punaruu, while now it exists also in one 
valley to the northward and in five more to the southward; the tide of immigration 
seems to have reached the most distant of the new areas between the years of the 
author’s third and fourth expedition. Like P. clara, therefore, P. nodosa has 
recently entered upon an epoch of renewed vigor, as manifested by its increase in 
numbers and in the area of its extent; previously it had become restricted to a small 
and confined territory, as in the case of P. filosa, and was in danger of extinction. 
The species is far from uniform throughout its range, as regards the secondary 
characters of the shells. Four varieties are herein established on the basis of 
geographical location and intrinsic differentiation; they are composita Cr. in Taa- 
puna and Punaruu Valleys, /eva Pilsbry of Maruapoo Valley (sznistralis of Mayer), 
exigua Cr. of Atehi Valley, and intermedia Cr. of Papehue, Aoua, and Orofere 
Valleys. ‘The first is predominantly dextral, the second is almost entirely sinistral, 
while in the fourth, reversed shells are produced only sporadically. Furthermore, 
four color-classes are distinguished, some of which have been given varietal names 
by earlier authors; they are pallidior Pilsbry, trilineata Pease, nodosa Pfeiffer, and 
concrescens Cr., all four of which may or may not be present in a given colony. The 
reasons are given at length for regarding the color-differentiation as subordinate, 
and that diversification which includes the element of geographical discontinuity as 
primary. 
On the basis of the full comparisons, it is contended that var. composita is 
directly ancestral to /eva and to intermedia; exigua has arisen from the latter only 
subsequently. 
The réle of the environment in the differentiation of the whole species seems to 
be negligible so far as causal values are concerned. The production of diverse 
color-types in one and the same valley could scarcely be attributed to the action of 
