1911.] NATURAL SCIENCES OF PHILADELPHIA. 177 



The canyon forms a barrier to the distribution of Oreohelix, the 

 widely spread Arizonian species 0. yavapai not extending north of it, 

 though very abundant in several subspecies on the southern side, 

 while 0. strigosa depressa, very abundant north of the canyon, does 

 not to our knowledge occur in the canyon or in Arizona south of it. 

 Some of the smaller species may prove to be similarly restricted, but 

 more copious data are required to prove that this is the case. The 

 minute species, here as elsewhere, are widely distributed, probably 

 owing to the facility with which they may be carried by cyclonic 

 storms. 



In a former paper of this series 2 we discussed briefly the relation 

 of desert snails to their environments, concluding that the direct 

 influence of desert climate had been overestimated basing this 

 opinion upon the fact that these animals are quiescent except during 

 the brief periods of damp or rainy weather; and accounting for the 

 opaque and chalky texture of exposed snails as a protection against 

 sunlight, probably brought about by selection. A recent letter from 

 Dr. Wm. H. Dall, giving his somewhat diverse views upon the same 

 topic, is here printed, by permission, so that those interested in these 

 questions from the standpoint of molluscan study may have both 

 views before them. 3 



2 Mollusca of the Southwestern States, IV: The Chiricahua Mountains, 

 Proc. Acad. Nat. Sci. Phila., 1910, pp. 47-50. 



3 " In regard to direct action of sunlight and other factors of climate on desert 

 snails (among which I reckon only those really exposed to it, and not those like 

 Ashmunella, which by descending into the rock piles reach a moderately 

 humid climate), my reasoning would be something like this: We know irritation 

 of the surface in snails causes exudation of mucous matter (mixed with lime in 

 the case of shell bearers), which tends to thicken and incidentally to contract 

 or corrugate new growth, this irritation may be alkali in fresh waters, sand or 

 infusoria in pearl oysters, alkali dust on arid windy volcanic islands, like the 

 Galapagos or St. Helena, and scorching sunlight in desert places. Now the 

 first result would be to thicken the shell and exclude the irritant, otherwise 

 the animal will die. Assuming that before reaching the point of absolute exhaus- 

 tion the amount of mucus has a limit, this means a retardation of growth in the 

 spiral direction, and if (assuming that the color glands have also a limited amount 

 of color to give the general secretion) it would be, in the case cited, abnormally 

 diluted; with the result that the shell would tend to be whiter than the normal, 

 not as a protection, but because of the dilution. This explains the white Bulimini, 

 Clausilias and Pupas, so conspicuous on hot rocks in South Europe. Then comes 

 in Natural Selection by killing off those which did not or could not sufficiently 

 thicken themselves to ward off the light, and you have by the most simple 

 direct action, without any heredity being called into play (unless through 

 some transmission of acquired characters, which I regard in this case as very 

 doubtful) all the characteristics of desert snails over the whole world developed 

 in the individual by direct action. 



"In the Galapagos snails the young nepionic whorls are normal, and I believe 

 would continue so except for the direct action of the environment. This affects 

 those on the ground, grass and low shrubs. Those living on the higher trees escape 

 (by my hypothesis) the dust and continue or remain normal in their growth." 



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