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THE AMEBIC AN NATURALIST [Vol. LI 



amount, larger amounts producing larger results" (1916, 

 p. 55). In crosses between animals possessing distinctly 

 ditferent degrees of the size "factor" blending rather 

 than alternative inheritance is supposed to result. This 

 also holds in the case of crosses between races differing 

 only slightly in the degree of the size character. In such 

 cases says Castle *'we do not find it easy to detect segre- 

 gation" (1916, p. 55).i 



The "second" view supposes that complex physiolog- 

 ical results such as the "size" of a certain animal depend 

 upon a complex of genetic factors. All these factors are 

 concerned in the growth and therefore the size of the 

 animal. The germinal determiners of these many factors 

 are considered to be units." As such their behavior in 

 inheritance is definite, non-blending, and essentially Men- 

 delian in nature. The blending, fluctuating nature of the 

 character studied is supposed to be due to environmental 

 factors, to the large number of hereditary factors in- 

 volved, and to absence or incompleteness of dominance. 



AVith this rather incomplete statement we may turn to 

 an examination of the experimental data. 



I. The Ixheritance of Spotting in Mice 

 Experiments on the inheritance of spotting in mice have 

 been carried on by the writer since 1909. As they pro- 

 gressed it became evident that spotting in mice offered 

 remarkable material for the investigation of the inheri- 

 tance of a variable character. Spotting as a character is 

 easily measurable and classifiable. It is not affected by 

 changes in the external environment. Variations in 



