No. 632] 



SABLE VARIETIES OF MICE 



261 



The results throughout indicate that we are dealing 

 here with genetic factors similar to those which have pro- 

 duced such quantitative differences as size in the races of 

 rabbits studied by Punnett. The results obtained from 

 crossing large and small races of rabbits agree in the pro- 

 duction in of animals genetically intermediate between 

 the parent races, and in the evidence of subsequent segre- 

 gation of the smaller size without segregation of the 

 larger. The same phenomenon appears in the present 

 study for light segregates have appeared but never the 

 dark. Placing the causes of darkness in mice in the same 

 category with the causes of size differences, which have 

 not yet been made clear, is an admission of the unsuitable- 

 ness of the material rather than of the insoluble nature 

 of the problem. Either the best material for the investi- 

 gation of such grade variations has not yet been found or 

 else the technique of observation and measurement of 

 such genetic differences as distinguished from non-her- 

 itable differences has not yet been evolved. The correct 

 interpretation of such differences must await an investi- 

 gation combining an optimum of material and method. 



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