PARTULA HYALINA. 49 
unfavorable for the species, the facts are unintelligible. To this case could be added 
many of the converse nature where, in the favorable region of the northern drier 
quadrant, hyalina colonies are numerically weak. If, then, the relation between 
abundance and a particular environmental condition like greater dryness or wetness 
is not invariable, the ultimate and primary causes of colonial vigor or weakness are 
to be sought in constitutional characteristics of the snails themselves. 
By a parity of reasoning, the diverse structural characters of the shells of 
hyalina, particularly in proportional measures, must be referred ultimately to con- 
genital differences in the animals, and not to the direct operation of differences in 
moisture or in supplies of food. It is theoretically conceivable that in a given 
valley the snails might feed more and consequently grow to a larger size, while in 
another region they might feed less and remain stunted in stature. But the facts 
rule out any such hypothesis. More rain and wetter forest render the snails active, 
so that they feed during more hours of the day in a moist region than in a large dry 
valley. Yet the shells of the drier north, though shorter, are wider and stouter 
than those of the wet southern quadrant. ‘The shells of Taiarapu as a whole, a 
region far wetter than Tahiti nui, are longer and wider but less stout as compared 
with those from the larger part of the entire island. ‘Taking all the facts into 
account, we can only conclude that congenital factors are ultimate for the establish- 
ment of the varying colonial types observed in different valleys. 
FECUNDITY. 
Like all other species of the genus, Partula hyalina is viviparous. The shell is 
about 3 mm. in diameter when the young animal leaves the brood-pouch of the 
parent and shifts for itself. The complete study of hyalina must deal with certain 
aspects of the relations between young and parents, although these relations are of 
far greater interest in other cases, like otaheitana and nodosa, where definite color 
varieties occur within the limits of the species, and where both sinistral and dextral 
shells are produced in one and the same species and variety. 
When the brood-pouch of a gravid individual is opened, the developing young 
are revealed, each inclosed within a tough egg-shell which, in this species and in one 
or two others (such as P. clara), isinvariably transparent. ‘The egg at the time of its 
fertilization is extremely minute; as the embryo grows, it feeds upon the albumen 
around it and gradually comes to fill the inclosing egg-shell. 
The number of eggs and of embryonic young varies in individuals taken at any 
one time, the highest being 9. They are arranged in order with the most advanced 
nearest to the birth aperture, that is, in a sequence according to age. In only a 
single instance out of the 250 records of gravid hyalina adults was there any departure 
from this order, in a case where the formula was as follows: y, y, y, egg, y, y, egg— 
the last egg being most internal. Such anachronisms are found with equal infre- 
quency in the other species of this island. 
The statistical data relating to the fecundity of hyalina in my collections are 
presented in table 15. It is obvious that a given individual does not breed continu- 
ously throughout its productive period, for only 64.9 per cent contain embryonic 
