76 MEMOIRS NATIONAL ACADE^IY OF SCIENCES. [vol.xii. 



conditions or permits the different kinds of shells to intermingle, cross, and thus diffuse through 

 the lo population certain germinal characteristics; or even to isolate or segregate them, because 

 the segregation of characteristics, as in individuals, may preserve differentiation. Long ago 

 Wetherby ('76, p. 4) suggested that in this family hybridization "may account largely for 

 variation in form." If we ignore the history which permits these germinal conditions and 

 their effects and study effects only comparatively we are particularly liable to attribute to a 

 single cause the results which are due to the influence of several causes. It is therefore only by 

 the history of each case that we may hope to make each conclusion stand upon its own merits. 

 There is thus the same need for considering the history of results produced in the past as there 

 is now for giving the history or method of procedure in a contempoi ary experiment. It is from 

 this standpoint that one of the main values of the history of the Tennessee River system is 

 derived. The changes in the drainage may be looked upon as just so many opportunities for 

 the diffusion or expansion of the hereditary characteristics of these snails as a means of reaching 

 new environments and as a means of remaining with or in the same environment. In this 

 manner is seen the biological justification of such facts and inferences. 



The development of the conditions which have permitted and caused the present diversity 

 and geographic range in lo show clearly that some of these influences are due to the responses 

 to the physical environment, while others are due to responses which different kinds of germinal 

 substances produce when isolated, or produce in response to one another. 



6. GEOGRAPHIC RANGE AND VARIATION OF EACH FORM. 



The different forms of lo not only show much local variation, but they also do in different 

 parts of the same stream and in different streams. To understand these it is necessary to recall 

 the races of forms which were indicated in an earlier chapter. The degrees of intergradations 

 which exist between these different forms make it very difficult or impossible to give their pre- 

 cise range. Some of the variations in the development of the shell, have been discussed in the 

 chapter on ontogeny of the shell, and others were when discussing the quantitative data. Here 

 we wish to consider the geographic range of each of the 14 forms recognized, and to observe 

 the variations of the shell which change with locality, and to indicate the affinities suggested 

 by such relations. Cf. Plate 1. 



Powellensis. — This shell is found almost solely in the headwaters of the Powell River. 

 Downstream below Pennington Gap it can not be recognized in abundance. Smooth sheUs 

 are found in lot 37, from Powell River station, and in lot 29, from Greens Ford. I have con- 

 sidered all the smooth shells in the Powell as 'powellensis. The optimum development of this 

 sheU is seen in groups 1 and 2. A few smooth, or relatively smooth, shells with low spines or 

 nodules, reach to near the mouth of the river. It appears that these shells with incipient spines 

 are due to crossing with the spinose population in which they occur. That these have not been 

 recently transported to these lower waters by floods and similar agencies, is suggested by the fact 

 that such shells tend to develop spinosity at an earlier age than those in the headwaters, and 

 can thus be distinguished from them in some cases. 



The small size and relative thinness of these shells is probably due in part, to the small 

 size of the stream and to the relatively small part of the headwaters which drains a limestone 

 region. The shells in groups 1,2, and 3 are strongly stained brown with iron, derived from the 

 sandstones, whUe the remaining groups of shells are not so staiaed, but are encrusted with 

 clay and algae. 



Lyttonensis. — This is a transitional form between the smooth and spinose kinds of shells 

 in the Powell. It occurs in the headwaters in smaU numbers, evidently pioneers which have 

 become established upstream in advance of the main body which predominate farther down- 

 stream, where their numbers increase imtil in group 2, at Lyttons MiU they are the most abun- 

 dant shell. In group 3 they appear to be almost the only form present, and below this point its 

 recognition is obscured. This form of shell appears to grade, in the lower Powell, very gradually 

 into the shells which are spinose throughout their post-embryonic development, or which become 

 spinose at the class 2 stage. These shells have been exceedingly puzzling and I have been 



