OP CONCHOLOGY. 



so many slightly varying forms, all closely allied with each other. 

 They are the more excusable because they have not perhaps 

 had opportunities to study these several forms in their vital rela- 

 tions to nature. A few years of patient inquiry in this direc- 

 tion would enlarge their minds to the perception of the fact that 

 we really have more species than are yet named. 



" I come to this conclusion by a Darwinian mode of reasoning, 

 taking into consideration the fact that we have shells distributed 

 over an immense territory, and in stations so diverse that the 

 continued influence of diversified conditions must result in perma- 

 nent modifications of perhaps a single original type. And I 

 must regard it as a law of diversification that once a race has 

 permanently developed characters in one direction different from 

 the characters of another race from the same stock, the two 

 cannot again converge. Hence we must have diversified species. 

 Time is the chief element in this work in connection with the 

 influence of external condition to which life must ever conform. 



" The same modes of reasoning may also apply to the various 

 Strepomatidce of this country. It may also be urged for Phjsa 

 and Lymncea. But, I have somehow the impression that, as 

 living entities, these last are a more modern development than 

 the operculated aquatic mollusca. Their comparatively recent 

 introduction may be urged as a plea for not multiplying species 

 unnecessarily, as in attempting to define the limits of species we 

 shall be more perplexed than with the operculated races. The 

 reasons are these : The forms of the species are less permanently 

 fixed. They blend with each other in a more confused manner, 

 and if we attempt to define a single species of a group, we shall 

 find in the next species of the same group specimens that we can- 

 not say are unlike the preceding. 



" During the last fifteen years I have studied Physa very 

 faithfully. Every little mountain stream, spring and rivulet has 

 its family. The family will have its peculiar features, and a 

 Rafinesque could make hundreds of species by going over the 

 limited area I have studied. Yet, when we trace the rivulets 

 along their beds, we shall find at every turn of the stream a new 

 feature. Here a streak of sunshine warms the water ; directly 

 a spreading oak casts a shade ; a little further on the ground is 

 marshy ; beyond that a spring of cold saline water from the 

 Utica slate makes new conditions ; and each of these modifica- 

 tions of one little rivulet has — what ? Shall we call them spe- 

 cies ? No. But it is an epitome of the whole continent ; and 

 over its vast territory we find our species. 



" In the spring, when the snows become a torrent, the bed of 

 the rivulet is washed clean of all its shells. A few that have 



