‘3 
1911.] NATURAL SCIENCES OF PHILADELPHIA, 177 
The canyon forms a barrier to the distribution of Oreohelix, the 
widely spread Arizonian species O. yavapat not extending north of it, 
though very abundant in several subspecies on the southern side, 
while O. 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 varried by cyclonic 
storms. 
In a former paper of this series’ 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.* 
2Mollusea of the Southwestern States, IV: The Chiricahua Mountains, 
Proc, Acad. Nat. Sci. Phila., 1910, pp. 47-50. 
2«Tn 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 willdie. 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 nepionie 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.” 
