448 THE AMERICAN NATURALIST [Vol. LII 



in harmony with the atomistic principles of physics and 

 chemistry. "Unit-factors" have even been identified 

 with molecules. 



In respect to the pigmental characters of our geo- 

 graphic races, it has been shown to be probable that in- 

 sensible gradations occur throughout considerable ranges 

 of territory. There results a series in which marked 

 contrasts can be found only by comparing individuals 

 from widely separated localities. The hypothesis that 

 the variations in this case are of the Mendelian tvpe 

 involves the assumption that the subspecific differences 

 have arisen by a whole succession of small mutations in 

 the same direction, the number of these mutations being 

 a function of the distance from some hypothetical center 

 of dispersal. In a recent paper (1917) Morgan has con- 

 sidered the question whether there are "any connections 

 between the gradations of character in allelomorphic 

 series and the order in which the characters appear," 

 %. e., whether "the assumed fluctuation of factors is a 

 sequential process." He concludes that, "as a matter 

 of fact, there is no such relation known ... for the ac- 

 tual evidence from multiple allelomorphs shows that 

 genes may mutate in all directions and also that extreme 

 mutations such as white eyes arise suddenly from red 

 and not by graded steps" (p. 524). These assertions, 

 which, it is true, were primarily concerned with the 

 effects of selection, lend little support to the view that 

 graded geographic variations have arisen through mu- 

 tation. 



The attempt to explain away the substantial mass of 

 evidence for permanent gametic blending and the indefi- 

 nite efficacy of selection by invoking the theorv of -mul- 

 tiple factors" is too well known to be reviewed here. 

 Castle has been the most able and vigorous opponent of 

 this theory. At present I will merely refer to certain 

 evidence of my own which. I think, supports such an ex- 

 planation no better than Castle's. 



The dorsal tail-stripe is entirely lacking in a certain 

 strain of my mutants. This stripeless condition i« reces- 

 sive to the striped one. In crosses with normal mice the 



