124 Shull. 



J\fus musculus (mouse): 



Piebald coat-color (Cuenot 1907, Lang 1911) 

 Mus rattus (rat): 



Hooded coat-pattern (data of Castle) (East 1912, Johannsen 1913, 

 Hagedoorn 1914). 



Until the plural determiners for any characteristic have been iso- 

 lated in different individuals, the actual similarity or difference of the 

 characters they produce and the method of their inheritance can not 

 be demonstrated. For all cases in which such analysis has not been 

 made the existence of plural Mendelian determiners must be purely 

 hypothetical; nevertheless, in the absence of other adequate interpreta- 

 tions for the phenomena recorded by these investigators, the existence 

 of plural Mendelian determiners should be accepted as a plausible work- 

 ing hypothesis. 



Granting the existence of such plural determiners, what is the 

 likelihood that any of them are also duplicate determiners? There is 

 no reason to suppose that a larger proportion of the independently in- 

 heritable factors of size, form, etc., are really duplicate, than of those 

 more easily analyzed characters whose Mendelian inheritance has now 

 been fully demonstrated. How many thousands of characters of various 

 kinds have been proved to follow the Mendelian method of inheritance, 

 I do not know, but certainly the four cases of fully demonstrated dupli- 

 cation of determiners represent an insignificant proportion of these, and 

 would, therefore, by themselves form an extremely slender basis on 

 which to rest the thesis that quantitative characters are generally 

 Mendelian in inheritance. It is only because such general features 

 as size and form and such physiological relations as length of vegetative 

 period and resistance to disease or to cold, may rest upon many quali- 

 tatively as well as quantitatively different elements, each of which 

 may be controlled, conceivably, by one or more Mendelian determiners, 

 that the hypothesis of the Mendelian inheritance of these features 

 becomes adequate. It ought to be emphasized, therefore, (1) that although, 

 historically, the Mendelian interpretation of the inheritance of size- 

 differences and of complex physiological capacities and activities has 

 been developed as a result of the discovery of duplicate determiners, 

 it need not have awaited that discovery, and (2) that this historical 

 connection must not be taken to indicate that the elements which make 

 up these complex characteristics, or the genes which produce or control 

 them, are in any case of duplicate nature, - - although, on the other 



