128 WHAT EVOLUTION IS 



sperm or egg^ can carry the exciter 

 or gene of only one of two opposing 

 characters, such for instance as white 

 and black. No germ cell can carry a 

 gene for white and a gene for black 

 at the same time. In any pair of op- 

 posing traits, the gene of only one 

 can be present in any germ cell. In 

 other words the germ cells are in this 

 respect always individually pure. 



This principle of the purity of the 

 germ cell makes clear the remarkable 

 proportions, already pointed out, in 

 the second generation of offspring. 

 It will be recalled in the example of 

 the guinea pigs that, in the second 

 descendent generation, there were one- 

 quarter or twenty-five per cent pure 

 whites, another quarter or twenty- 

 five per cent pure blacks, and a half 

 or fifty per cent black individuals 

 which, however, were really mixed, 

 for, on being bred amongst them- 



