THEORY OF EVOLUTION 83 



arisen in nature and differ definitely and strik- 

 ingly from each other, yet any two differ by 

 only one factor difference. 



Similar relations have been found in a num- 

 ber of domesticated races. In mice there is a 

 quadruple system represented by the gray house 

 mouse, the white bellied, the yellow and the 

 black mouse (fig. 44). In rabbits there is 



\ 







FIG. 44. Diagram illustrating four allelomorphs in mice, viz. 

 gray bellied gray (wild type) (above, to left) ; white bellied 

 gray (above, to right) ; yellow (below, to right) ; and black 

 (below, to left). 



probably a triple system, that includes the al- 

 bino, the Himalayan, and the black races. In 



