48 VARIATION, DISTRIBUTION, AND EVOLUTION OF THE GENUS PARTULA. 
constitute the longest group of the whole island, while in Tautira they are almost the 
shortest collectively. The proportionate measurements vary quite as much as the 
absolute dimensions, and independently of the latter to some degree. In general, 
shortness and stoutness of the whole shell go together, but the Tipaerui shells, as 
compared with those of Fautaua, are far more slender, even though they are shorter 
as a group. Each colony, therefore, seems to be more or less peculiar in its own 
way—in an absolute dimension, ina proportionate characteristic, and in its particular 
combination of all seven determinations. 
INTERPRETATION OF THE FACTS. 
The observations recorded in the foregoing sections render it possible to effect 
a partial solution of the major problem under consideration—the problem, namely, 
which deals with the causal value of the “environment” in producing varietal 
modifications wherever these can be demonstrated. We have seen that, in Tahiti, 
Partula hyalina occurs in secluded colonies living in more or less rigidly isolated 
valleys, in practically all of the administrative and geographic districts of the island. 
But the species is far from uniform, either in its frequency or in the structural 
characteristics of its shell. Must we conclude that the observed differences owe 
their origin to diverse environmental conditions obtaining in different areas of the 
island? In my opinion, the question should be answered in the negative. 
In the first place, it is impossible to find geological, meteorological, or hetero- 
generic biological conditions which vary essentially around the island. The plants 
upon which the animals are found occur everywhere, although their relative abund- 
ance is by no means the same in widely separated or even in neighboring and parallel 
valleys. As the snails eat decaying vegetation, it is natural that no correlation can 
be demonstrated between the prevalence in a given valley of a certain type of habit- 
plant, like the Colocasia, and certain characters of hyalina in that valley, as regards 
either colonial abundance or shell structure. The facts demonstrate that hyalina 
occurs more numerously in the drier northern district and in the larger valleys of 
this and other quadrants. It is also true that the shells of the northern drier section 
are different morphologically from those of colonies in the wet regions. But the 
facts also show that the relations mentioned are not invariable by any means; if 
they were, we might be forced to conclude that high and low degrees of moisture 
acted as direct causal factors for the development of peculiar varietal features. 
The conclusion which appears more just is that the morphological differences 
observed are due to spontaneous congenital causes that remain unknown in them- 
selves, but whose effects are produced quite independently of the external conditions; 
to this statement we may add that even the greater abundance of a colony found in 
a drier valley may also be indicative of a congenital variation of a favorable nature. 
With regard to the latter point, it may be recalled that in such a valley as Otuna, 
situated in the wet district, yal/ina forms more than 5 per cent of the population as 
represented by 382 adult snails; in Maruia, the nearest valley on the west of Otuna, 
hyalina does not occur at all. Unless this indicates a congenital vigor of the snails 
of the Otuna colony, making them fit to meet circumstances that on the whole are 
