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. 10 



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. 



III. Factors 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. 11 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 



10 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. 



11 In White Tail Canyon, however, Sonorella micro, 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. 



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