Xos. 620-621] INHERITANCE IN PEEOMYSCUS 



147 



"grizzled" heads likewise to the local loss of hair pig- 

 ment, while the white snouts have resulted from a loss of 

 pigment both in the skin and hair of the latter region. 

 The single exception among the "mutations" which I 

 have observed in Peromyscus is the occasional presence 

 of skin pigment in the tail. Here something has been 

 definitely added to the usual condition. 30 



In heredity, too, these mutant characters, whether nega- 

 tive or positive, behave in distinctly discontinuous fash- 

 ion. They do not blend, but are either present or absent 

 in their entirety. 



Taken at face value, I say, the evidence shows that we 

 have to do here with two different types of variation and 

 Two different types of heredity. Now admittedly, the 

 naive view of such a situation is not necessarily the cor- 

 rect one, else we should be forced to return to the geocen- 

 tric theory of the solar system. But even in this last in- 

 stance, the burden of proof most assuredly rested on the 

 man who first asserted that the sun did not move around 

 the earth. And to-day the same burden rests upon those 

 who claim— possibly with truth— that heritable variations 

 are all discontinuous and that blended inheritance is an 

 illusion. 



In the few remaining pages of this paper, it is obviously 

 impossible to discuss the various lines of evidence which 

 have been advanced in favor of the Mendelian-mutation- 

 pure-line scheme of things. I think that few would be 

 enthusiastic enough to assert that the case had yet been 

 really proved on evidential ground-. The considerations 

 which are chiefly effective in determining one's adherence 

 to this system of beliefs are doubtless of a more general 

 nature. Thus it is argued that Mendelian inheritance has 

 been shown to hold rigidly throughout a vast range of 

 material, and that, therefore, the "unit-factor" concep- 

 tion is probably of universal application. Or, it is con- 

 tended that the scheme of things here considered is more 



