44. VARIATION, DISTRIBUTION, AND EVOLUTION OF THE GENUS PARTULA. 
also summarized in a condensed form, so as to bring out more clearly any existing 
relation between an ecological character and a general degree of relative abundance. 
Two conclusions are warranted by the facts as given. The first is that the drier 
northern quadrant is most favorable for hyalina, because no valley which bears any 
snails at all is devoid of this form, while in 6 of the 10 valleys hyalina forms more 
than 2 per cent of the Partula population. The western sector is next in salubrity 
and is followed by the relatively moist eastern section. “The wettest series of south- 
ern valleys proves to be the least favorable, for only 3 out of 20 valleys have more 
than 2 per cent of this species in their snail population. 
The second conclusion is that the /arger and drier valleys are more favorable 
than smaller elements with a higher degree of humidity. For example, in the 
northern quadrant the great valleys of Fautaua and Papenoo have the highest 
percentage of hyalina, 11 and 7 per cent respectively; the valleys of the next order 
(Ahonu, Tuauru, Pirai, and Tipaerui) average nearly 3 per cent (2.98), while the 
four tertiary valleys of the list average only 1 per cent, a figure that is brought up 
by Hamuta, which, possibly, is influenced by its close proximity to Fautaua. 
In brief, the drier localities appear generally to be more favorable for the species 
than other regions. This result is of much importance ia connection with later 
discussions of hyalina as it exists in the Cook and Austral Islands. 
The facts, however, do not warrant the assumption that the relation of this 
species to certain environmental conditions is fixed and invariable. Many dis- 
cordant cases appear from the full table (table 9), and some of these have been 
already specified. We are justified in stating that high degrees of general humidity 
are on the whole unfavorable to the species, but the exceptions are sufficiently 
numerous to demand a qualification of this assertion. It must be recognized that 
the vigorous and numerous colonies which exist in small valleys like Otuna, in the 
heart of an unfavorable region, must differ constitutionally from the generality of 
the species that are not so able to withstand adverse conditions. Hence, the 
conclusion warranted by the whole series of facts relating to the distribution of 
colonies of hyalina that vary in numerical abundance is as follows: The members of 
this species vary constitutionally in diverse localities so as to possess different degrees of 
ability or disability to meet the particular conditions obtaining in such diverse localities. 
Thus the observed relations are due primarily to ultimate factors of a constitutional 
and congenital nature, and not to external conditions alone. 
THE SHELL OF PARTULA HYALINA AND ITS VARIATION—TAHITI. 
Passing now to the statistical study of the structural characters exhibited by 
the shell of this species, we come to a more direct analysis of the problems of dis- 
tribution and evolution, to which indeed the foregoing discussion of numerical 
distribution has been introductory, though no less significant in its result. We 
may now deal with the morphological peculiarities of the individuals which make 
up the colonies found in the various valleys, in order to ascertain whether local 
variation occurs; and, if so, whether this can or can not be related to environmental 
differences as causal factors. 
