240 HEREDITY AND ENVIRONMENT 



hybrid parents must be found in the composi- 

 tion of the male and female sex cells. Since 

 such extracted dominants and recessives breed 

 true, just as pure species do, it must be that 

 their germ cells are pure. In the cross be- 

 tween pure races of white-flowered and red- 

 flowered Mirabilis the germ cells which unite in 

 fertilization must be pure with respect to white 

 and red, though the individual which develops 

 from this cross is a pink hybrid. But the fact 

 that one-quarter of the progeny of this hybrid 

 are pure white, and another quarter pure red, 

 and that these thereafter breed true, proves 

 that the hybrid produces germ cells which are 

 pure with respect to red and white. Further- 

 more the fact that one-half the progeny of this 

 hybrid are themselves hybrid may be explained 

 by assuming that they were produced by the 

 union of germ cells carrying pure white and 

 pure red, as in the parental generation. 



Mendel therefore concluded that individual 

 germ cells are always pure with respect to any 

 pair of contrasting characters, even though 

 those germ cells have come from hybrids in 

 which the contrasting characters are mixed. 



