50 PROCEEDINGS OF THE ACADEMY OF [Feb., 
distinct species live under conditions which so far as we can see are 
identical. The subterranean Sonorellas, such as bicipitis, optata, 
bowiensis and leucura inhabit slopes having similar exposure, vegeta- 
tion, elevation, and rock cover, yet they differ characteristically in 
genitalia. At the same time, one style of genital organs seems to be 
as fit as another. Nearly all of the Holospiras live in very similar 
places. The local differences in the stations of the typical colonies 
of Oreohelix chiricahuana, obsoleta and percarinata are apparently 
less than the differences between the several stations of colonies of 
chiricahuana. If the conditions seem practically identical on parallel 
slopes of different canyons, it seems hopeless to search for differential 
conditions among several rock-piles or taluses along one canyon side, 
having the same exposure, composed of the same rocks, separated 
perhaps only by arid gulleys or barren ‘earth slopes; yet such taluses 
often harbor perceptibly different colonies of the same species. 
The facts developed in our Arizona work lead us to doubt the 
potency of environment as a direct agent in effecting specific. differ- 
entiation, or at least to assign such factors a wholly subsidiary role. 
The facts seem explicable only on the hypothesis of variations existing 
or arising in the constitution of the egg, leading to modifications of 
the adult organism which for the greater part are indifferent as affecting 
the well-being’of the race. Such adaptation as exists would apparently 
be due to selection. The isolation of small colonies in these moun- 
tains must favor the survival of what are currently called mutations 
occurring therein. The occasional mingling of neighboring colonies 
in which diverse variations have arisen seems to have led to such 
heterogeneous colonies as we have described in Holospira.” 
We have no definite evidence to offer bearing upon the amplitude 
of individual variations or ‘‘mutations,’’ yet we may again mention 
the fact that in many species nearly every colony has its minor pecu- 
liarities—peculiarities far below the grade usually called “specific,” 
yet appreciable to the trained eye.’* It may be inferred from this 
fact that the amplitude of “mutations” is ordinarily not great. 
IV. DiIstTRIBUTION OF CHIRICAHUAN MOLLUSKS. 
The snail fauna of the Chiricahuas consists of 51 species, falling into 
two main categories: 
12 We believe this to be the explanation of the diversity of colonies in the 
polychromatic arboreal snails such as Partula and Liguus, in which some colonies 
of a given species are homogeneous, while others are heterogeneous, snails of 
several definite color-patterns being the offspring of a single mother. 
3 Only the application of careful biometric methods can fully or definitely 
bring out these minute differences. 
