INHERITANCE OF MENTAL DEFECTS AND DISEASE 69 



Doubtless the writers who attribute so much to degeneracy- 

 have often failed to recognize traits which are separately trans- 

 missible. But on the other hand, exclusive attention to the 

 inheritance of particular characteristics leads to a disregard of 

 other features of organisms which may be associated with the 

 characters studied. Most studies made upon the Mendehan 

 inheritance of human traits suffer from this drawback. Inspired 

 by the desire to apply Mendel's law to all heritable traits, Mendel- 

 ians have focussed their attention almost exclusivily upon partic- 

 ular characters in the hope of unravelling the complex skein of 

 human inheritance by tracing out the individual traits. With 

 fuller experience with Mendelian phenomena it is coming to be 

 recognized by many investigators that ''characters" are not 

 entities by themselves, but symptoms of general and deep-seated 

 though it may be slight modifications. As Dr. T. H. Morgan says : 

 "Most students of genetics realize that a factor difference usually 

 affects more than a single character. For example, a mutant 

 stock [of Drosophila] called rudimentary wings has as its principle 

 [principal] characteristic very short wings. But the factor for 

 rudimentary wings also produces other effects as well. The fe- 

 males are almost completely sterile, while the males are fertile. 

 The viability of the stock is poor. When flies with rudimentary 

 wings are put into competition with wild flies relatively few of the 

 rudimentary flies come through, especially if the culture is 

 crowded. The hind legs are also shortened. All these effects are 

 the results of a single factor-difference." Such flies may be called 

 degenerates; whether they are more variable than robust races 

 we do not know. 



There is no doubt that many writers of a generation or more 

 ago employed the notion of degeneracy in too wide and loose a 

 sense. Nevertheless there may be an important element of truth 

 in the idea which is apt to be overlooked by modern geneticists in 

 their preoccupation with the transmission of particular and clearly 

 definable characteristics. A more critical study of degenerate 

 strains of plants and animals might afford valuable suggestions 

 for the interpretation of many phenomena of human heredity. 



