1910. ] NATURAL SCIENCES OF PHILADELPHIA. 49 
Canyon. Where snails are gathered from one limited region and 
those from various colonies are not kept separate, the measurements 
of such series when plotted may form conspicuously bimodal curves, 
due to the mixture of shells from different exposures. Such results 
are entirely worthless in the study of the relations of organism to 
environment. 
We have made no exhaustive series of measurements to ascertain 
whether the height of the spire varies with elevation of the station, 
but such observations as we have made indicate that it does not. In 
the helices there is rather wide individual variation in height of spire 
at all levels indifferently.'° 
Influence of the Character of the Rock.—Land snails are notoriously 
more abundant on limestone than where the country rock is igneous. 
They are also usually heavier, the shell-walls thicker, though this is 
not the case with all species. The individuals apparently reach 
at least as large a size where lime is scarce as where it is abundant, if 
other conditions (exposure and humidity) are about equal. In several 
cases the largest individuals occurred under granitic rock, as in the case 
of Sonorella bicipitis at Nine-mile water-hole. 
Enemies.—Field mice are apparently the chief enemies of the larger 
snails of the Chiricahuas. We often found unmistakable evidences 
of their destructive activity. The crevices of rock-piles which harbor 
most of the snails are often accessible to mice; and no protective 
device seems to have arisen effective against the latter. Oreohelix 
barbata, which from the dirt on its hairy coat is rather hard to see, 
we noticed on several occasions had been cruelly preyed upon by 
mice. We can offer observations upon predacious insects. 
Ill. Facrors In THE FoRMATION OF RACES AND SPECIES. 
The several modifications of the shells which we have noted above 
as correlated to some extent with external factors are only in minor 
part such features as serve to signalize species."' Thus in Sonorella 
the species are based mainly upon characters of the genitalia. In 
Ashmunella upon the teeth of the aperture and the shape of the last 
whorl. In Holospira the shape of the spire and its sculpture are the 
chief differential features. Moreover, in many cases, allied but 
* Bellini’s recent claim that on Capri the snails from greater elevations have 
higher spires, due to diminished atmospheric pressure, seems to us fantastic in 
the extreme. 
“In White Tail Canyon, however, Sonorella micra and Ashmunella lepiderma 
differ from their fellows on the more shaded side of the canyon by features 
mainly traceable to the different exposure and rock. 
