162 THE THEORY OF EVOLUTION 



striped, but the yellow forms again split in the next 

 generation. Inasmuch as the sexual method of re- 

 production was independently acquired in both the 

 vegetable and the animal kingdoms, not inherited 

 from an ancestor common to both, this agree- 

 ment indicates that the Mendelian laws of inheri- 

 tance are of universal validity, save under such cir- 

 cumstances as will be mentioned later. 



The Mendelian laws further explain how new 

 races may arise through the sudden loss of one or 

 more factors by mutation. The common house- 

 mouse in its wild state has a characteristic gray 

 colour, but there are several fancy breeds of mice, 

 white, brown, black and blotched, all of which were 

 derived from the wild form. Many experiments in 

 the cross-breeding of these domesticated varieties 

 with one another and with their wild ancestor, in all 

 sorts of combinations, have shown that the varieties 

 have arisen through the suppression of factors. 

 Many factors are involved in these cross-breedings, 

 but, simplifying the matter to the utmost, they may 

 be reduced to four. (1) A colour-factor, which must 

 be present if any colour at all is to be developed in 

 the hairs of the coat. (2) A factor which determines 

 that the colour shall be distributed over the whole 

 body. (3) A pigment-factor, which conditions the 

 occurrence of black in the hairs, which, without it, 

 would be brown. (4) A distribution-factor, which 

 controls the arrangement of black and brown in each 

 hair, so that the combined effect is the characteristic 



