142 THE AMERICAN NATURALIST [Vol. XL VI 



Salmo, have become notorious and are not infrequently 

 referred to as proof of the futility of systematic work. 

 Does the discovery of biotypes afford any help in explain- 

 ing the existence of such genera? 



I think that it does, particularly when considered in 

 connection with the broadest interpretation of Mendel's 

 law. If we compare one of these inclusive genera with 

 one which contains few and well-defined species, we see 

 that the essential difference lies in the latter having the 

 characters sharply denned, with little diversity and no 

 blending, while in the former the same or similar char- 

 acters show so much diversity and such a tendency to 

 blend that the resulting recombinations are most perplex- 

 ing. It has occurred to me that we have here a condi- 

 tion of affairs analogous to what we find in the develop- 

 ment of the individual. Certain individuals with unlike 

 parents show what seems to be a blending of the parental 

 characters, while in numerous other cases the characters 

 of the individual can be referred unhesitatingly to one or 

 the other parent. Thus, as the well-known investigations 

 of Castle have shown, if lop-eared rabbits are crossed 

 with rabbits having ordinary ears, the character of the 

 ears in the offspring can not be referred to one parent 

 rather than to the other, while if pigmented and albino 

 rabbits are crossed, the color-character of the offspring 

 in succeeding generations can be so referred without 

 difficulty. This difference has been interpreted by Dav- 

 enport and others as due to the potencies of the deter- 

 minants, the apparent blending being associated with 

 equipotency or an approach thereto, while the distinct 

 characters result from allelopotency. Now may it not 

 be that a similar inequality of potency occurs among 

 the biotypes which go to make up a species? And so 

 when reproduction takes place we find some species in 

 which well-defined characters are dominant and the re- 

 sulting individuals form easily recognized groups, while 

 in other species there is a lack of definiteness and a blend- 



