THE VARIATIONS AND ECOLOGICAL DISTRIBUTION OF THE 



SNAILS OF THE GENUS 10. 



INTRODUCTION. 



In this paper are presented the results of a study of the variations and ecological distri- 

 bution of the river snails of the genus lo. The great amount of variation in these shells is as 

 striking as that found in the famous Slavonian Paludinas, studied by Neumayr ('75), or the 

 Planorbes of Steinheim, studied by Hilgendorf ('66, '01) and Hyatt ('80), and while these 

 fossils are found in different strata, and probably therefore are of diverse ages, lo is found 

 living to-day in a single river system, that of the Tennessee. 



Since this investigation began the point of view of naturalists has undergone several 

 important changes. The older conception of the species as the unit for study has been con- 

 stantly undergoing disintegration, as the significance of local races, varieties, colonies, pure 

 strains, characters, factors, and environmental influences has been enlarged as the result of the 

 recent investigations of variation, heredity, and more recently with the important advances 

 made in ecology. As the study of lo advanced it progressively became less and less a study 

 of variation and taxonomy in the older sense, and more and more of a study of the relation of 

 lo, to its complete organic and inorganic environment; or, in other words, it became more 

 ecological. 



In spite of its limitations I hope the present study will help to make concrete an idea so 

 well expressed by Brooks ('06, pp. 75-76): 



Inheritance and variation are not two things, but two imperfect views of a single process, for the difference between 

 them is neither in living beings nor in any external standard of extermination, but in the reciprocal interaction between 

 each living being and its competitors and enemies and the sources of food and the other conditions of Ufe. * * * 

 You will note that it is as great an error to locate species in the external world as it is to locate it in germ cells or in 

 chromatin. It neither exists in the organisms nor in the environment, because it is in the reciprocal interaction 

 between the two. 



In harmony with this ecological point of view an effort has been made to study these 

 shells in such a manner as to show the reciprocal responses of the changing environment and 

 the changing shells, because until both variables are studied and related little symmetrical 

 progress can be made. Our problem is more and broader than the origin of the differentiations 

 found, for it is concerned as well with the conditions of their survival and perpetuation where 

 they now live. For this reason the development of the environment is considered an essential 

 part of this study, a phase which the students of variation and heredity seldom consider very 

 fully. This study has been devoted mainly to the interpretation of the conditions found in 

 nature rather than from the standpoint of the student interested mainly in the immediate 

 control of nature. 



This group was chosen for study because of the large size of the sheUs, their great varia- 

 bility, and their relatively limited geographic range. They are confined solely to the Tennessee 

 River system, and mainly to that part which lies upstream from Chattanooga. With such a 

 limited distribution it seemed possible to cover entirely the geographic range, and thus secure 

 a certain completeness which it is very difficult or impossible to secure in many wide ranging 

 kinds. An examination of the material in some of our largest museums revealed at once the 

 necessity of gathering fresh material for this study, because of the lack of adequate data with 

 the collections previously made. To make these collections it was necessary to go on expedi- 

 tions for them for three consecutive years during the late summer and fall, when the streams 



7 



