WHAT EVOLUTION IS 127 



hidden in their bodies the white trait, 

 for, when they are bred amongst 

 themselves, one-fourth of their off- 

 spring are w^hite. Black, then, in 

 some way overcomes white; not that 

 it obliterates the white, but it holds 

 this trait in abeyance. In the language 

 of the modern breeder black is said, 

 in a case such as the guinea pig, to be 

 dominant over white and white is 

 said to be recessive to black. This 

 state of affairs, though not universal, 

 is common to many such pairs of 

 breeding characters. As a generali- 

 zation, it is often referred to as the 

 principle of dominance and was one 

 of the discoveries of Mendel. 



A second and very much more im- 

 portant principle, that is illustrated 

 by the example under consideration, 

 is, what may be called, the principle 

 of the purity of the germ. It is 

 briefly this: a given germ cell, be it 



