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. 
