THEORY OF EVOLUTION 



83 



arisen in nature and differ definitel}' and strik- 

 ingly from eaeli other, yet any two differ by 

 only one factor difference. 



Similar relations have been found in a num- 

 l)er 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. Diagi-iiiii illustratinu- fimr alli'limioriili.s in mice, viz. 

 gi-ay liellied g'ray (wild tyiu-) (aliove, tn left); wliite liellieil 

 gray (ahiive, to right); yellow (lielow, to right); and blaek 

 (below, to left). 



]H-obal)ly a triple .system, that includes the al- 

 bino, the Himalayan, and the black races. In 



