PARTULA HYALINA. SI 
three distinct classes of individuals, namely, the embryonic, the adolescent, and the 
adult snails. ‘The first-named constitute an arbitrarily defined generation destined 
to enter upon a free and independent existence at a later time and to become reduced 
in numbers through the inability of some individuals to meet successfully the condi- 
tions of adolescence and maturity. The second class of adolescents also constitutes 
an arbitrarily defined generation produced earlier, and also fated to be decimated 
before all its members can reach reproductive maturity. The adults, finally, form 
a third generation whose members are survivors of a still longer period of natural 
elimination. In the present material the third class is made up of the actual parents 
of the first class; but if the assumption is correct that the three specified groups are 
representative of an equilibrated population, the relationship in question is not a 
disturbing factor. 
In order to obtain a measure of the severity of elimination, it is necessary only 
to compare the known number of adults with the known number of embryonic 
young, for although the problem of the relative numbers of the post-embryonic but 
immature individuals is interesting, it can be disregarded in this connection. 
Judging from the actual facts at hand, 969 eggs and young are to replace 397 
adults. The exact number of young per adult snail is 2.44, as noted in the preceding 
section; or, in other words, 2 adults of one generation bring forth about 5 young 
of asucceeding generation. But unless the species is rapidly increasing in numbers— 
for which there is no evidence—then only 2 of the 5 new-born snails will survive 
to assume full adult form and ability, and 3 are destined to be eliminated through 
constitutional incapacity or from the effects of adverse external circumstances, 
such as prolonged drought, or from the attacks of other organisms. 
PARTULA HYALINA FROM THE COOK ISLANDS. 
The Cook Islands are seven in number and form a group centering about a point 
21° S. lat., and 158° W. long. They are thus about 500 miles to the WSW. of 
Tahiti and the same distance to the WNW. of Rurutu in the Austral Group. The 
principal islands are Rarotonga, Mangaia, Moki, Aitutaki, and Atiu. I personally 
obtained hyalina on Mangaia and Moki, while search was futile in Aitutaki and 
Rarotonga, although the last-named is reported to be inhabited by this species; I 
believe, however, that the record is erroneous. 
Rarotonga is the largest member of the group, and the only one that consists of 
volcanic rock. In its general structure it closely resembles Tahiti and other “high”’ 
islands, in so far as the whole mass is furrowed by deeply cut valleys, bearing 
vegetation essentially similar to that of Tahitian localities. In certain sections, 
however, cultivation has extended so greatly as to reduce materially the territory 
occupied by primitive forests in the floors of the valleys, even at some distance 
inward from the coast. 
Mangaia is an uplifted mass of coral limestone, which still displays the struc- 
ture of an atoll (plate 164.) A dry circular valley with high and abrupt walls occupies 
the interior, which was undoubtedly the lagoon of the island during an earlier 
