312 ' ORTMAN— FRESH-WATER MUSSELS. 



consequence of reaction to local environmental conditions, is 

 favored ; in the second case, no such local development is possible, 

 any tendency toward it being promptly obliterated by the mixing of 

 the different stocks. 



This, however, requires additional investigation. In most of the 

 species, chiefly those from the upper Tennessee region, we do not 

 possess sufficient information as to the fish host and its ecological 

 habits. My chief purpose in the present paper was, to bring out 

 the evidence for the actual existence of the law. 



