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 



