134 WHAT EVOLUTION IS 



fertilization. Once in four chances 

 a white sperm will fertilize a white 

 egg, a process which will yield the 

 twenty-five per cent pure white guinea 

 pigs. Once in four a black sperm 

 will fertilize a black tgg and thus will 

 arise the twenty-five per cent pure 

 black guinea pigs. Once in four a 

 white sperm will fertilize a black egg 

 and once in four a black sperm will 

 fertilize a white egg and these two 

 classes taken together will yield the 

 fifty per cent mixed stock which, like 

 their parents, can produce either 

 white or black offspring. Thus the 

 assumption of the purity of the germ 

 leads to a simple and illuminating 

 understanding of the proportionate 

 numbers of young in the several Men- 

 delian classes. 



This assumption has been tested by 

 geneticists in many ways and has been 

 found to hold good. In fact, the 



