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 cMricakmna, obsoleta and pcrcarinata are apparently 

 less than the differences between the several stations of colonies of 

 chincahuana. 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. 12 



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. 13 It may be inferred from this 

 fact that the amplitude of "mutations" is ordinarily not great. 



IV. Distribution 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. 



13 Only the application of careful biometric methods can fully or definitely 

 bring out these minute differences. 



