280 NORTH AMERICAN FAUNA. [No.7. 



The suggestion that arises from the study of the forms above re- 

 viewed, aud the regions and conditions to which they are related, point 

 to the causes that induce variation, and to the permanency of species 

 and genera, or to the mutability of the same, as dependent on environ- 

 mental factors, forces, or conditions. If we are warranted in assuming 

 or to indulge in the speculation, that with volume of water ample or 

 maximum and chemical proportions as related to volume minimum our 

 Tryonias would be smooth; and that the smooth form that so largely 

 prevails or dominates in the various species of the Bythinellas and 

 related groups is in a conventional sense of the word, normal, then we 

 may reasonably assume that upon the reversal of these conditions which 

 are environmental and apparently fundamental, with volume of icater 

 mimimum and with chemical proportions as related to volume of water 

 maximum, these phenomena of variation may be attributed, because 

 they are so generally coincident with the latter or alternative charac- 

 ter of the environment, though temperature conditions probably have 

 more or less influence.* 



At times, no doubt, the flow of water from the springs where these 

 forms occur is comparatively excessive, and there follows a limited lo- 

 cal extension of distribution or occupancy in the immediate region, 

 equal to the area covered by the overflow. With the decline of the 

 waters and the evaporation or drying up that follows, the larger areas 

 are inhabitable for awhile, as the mollusks of this general group possess 

 remarkable vitality, and can live for a long time away from, or with- 

 out water, in damp mud, by burying themselves below the surface. 



The soil or mud in the immediate region of alkaline or saline springs, 

 through repeated overflow and evaporation, becomes supersaturated 

 with the bitter chemicals, and it would seem that in course of time 

 these conditions might play some part in inducing variation in the 

 progeny of those individuals that possessed sufficient vitality to sur- 

 vive or to adapt themselves to these conditions. In many places, it is 

 not unreasonable to suppose that such or similar conditions are an ever- 

 present and operative influence within the environment. 



I have heretofore f called attention to the remarkable variation exhib- 

 ited by the pond snails, Physa, of the Colorado Desert, so abundant in 

 and around Indio. In these the sculptural feature has no part, but the 

 forms present not only the normal aspect of several well-known species, 

 but the varietal phases, furnish connecting links between them, as well 

 as extraordinary extremes to the extent of distortion. 



Now these alternations of conditions are exactly what have occurred 

 within the vast area, in various places of which, these forms occur. 



No doubt there are many other springs still living (flowing) within 

 the general region that await examination. The territory inhabited by 

 Gould's species includes not only the localities from whence Merriam, 



*Iii this connection see Call's interesting and able paper "On the Quaternary and 

 Recent Mollnsca of the Great Basin," etc., Bull. 11, TJ. S. Geol. Survey, 1884. 

 tAm. Naturalist, October, 1883, pp. 1014-1020. 



