54 VARIATION, DISTRIBUTION, AND EVOLUTION OF THE GENUS PARTULA. 
GENERAL DISCUSSION OF THE DISTRIBUTION OF PARTULA HYALINA. 
On the basis of the details and summaries presented in the foregoing pages, we 
may now undertake a broader discussion of the wide geographical range of Partula 
hyalina, which renders this species unique in its genus. The primary fact is that 
one and the same species exists in far-separated islands belonging to three distinct 
groups, and that these islands differ also in geological structure and origin. Tahiti 
is a veritable classic as an example of the so-called “high” volcanic islands. In 
the Austral or Tubuai Group, Rurutu (Oheataroa of Broderip, Oheatoroa of Cook) 
is about 8 miles in circumference and rises to a height of 1,500 feet; the lowermost 
level up to an altitude of about 100 feet is composed of coral limestone exposed by 
the uplift of the whole land-mass. ‘This island lies more than 250 miles from the 
nearest point of Tahiti, from which it is almost due south. ‘Tubuai is another island 
of the group, 100 miles southeast of Rurutu, and this also bears hyalina, as asserted 
by Garrett (Proc. Acad. Nat. Sci. Phila., 1879), who indeed regarded this group 
as the “metropolis” of the species. The geographical features of the Cook Group 
have been described in the foregoing section. 
The snails under discussion existing in these three groups obviously belong to 
the same species, even though the shells from the Cook Islands prove to be character- 
istically different in certain details from Society Island shells, when examined by 
precise statistical methods. We are not confronted by a case of parallel or con- 
vergent evolution, because it is not probable that convergence would bring about a 
double or triple development of colonies possessing so many identical qualities in 
texture and form of the shell, in the obliquity and shape of the aperture, and in the 
transparency of the embryonic membrane. ‘The differences noted are quantitative 
within the qualitative limits of the common characteristics, and there is but one 
natural species of P. hyalina. 
While the occurrence of one identical species in three distinct groups of widely 
separated islands is sufficiently remarkable in itself, it is still more surprising to find 
that it exists only in certain members of each of these three groups. Moorea lies 
only about 12 miles from Tahiti, but it is quite as free from hyalina as Raiatea and 
the more distant members of the Society Group. The absence of this type from 
Atiu and Aitutaki is not so unexpected; for these islands are flat, uplifted atolls, 
devoid of the dense jungle overshaded by high, precipitous mountain ridges which 
are such important environmental elements in Tahiti and its kind. Nevertheless, 
Mangaia and Moki are like Aitutaki, yet they bear hyalina; while Rarotonga, which 
is a high island, seems to be at least doubtful as a habitat for this species. 
There are two interpretations of these facts and only two: (1) hyalina may have 
been transported from its original locality in one of these groups to the other two, 
by human agency; (2) it may be an ancient species which had reached its present 
specific condition before the old and continuous mountain ranges had been isolated 
by subsidence so as to become the present series of unsubmerged peaks which con- 
stitute the different groups of volcanic islands. We may consider the facts pro and 
con in each case. 
