VARIATION AND HEREDITY 185 



interpretation or rationale of the facts. He 

 made the supposition that the generative 

 cells or gametes produced by the cross-breds 

 (FJ are of two kinds, each kind bearing only 

 one of the two contrasted or alternative 

 characters, which, as we have seen, do not 

 blend. He supposed also that the two kinds 

 are produced in approximately equal numbers. 

 Now if each of the hybrids of the P l generation 

 produces in both sexes 50 per cent, of its germ- 

 cells bearing the dominant character and 50 

 per cent, bearing the recessive character, then, 

 if fertilization be fortuitous, 25 per cent, of the 

 fertilized egg-cells will bear only the dominant 

 character, 50 per cent, will bear both the 

 dominant and the recessive character (only 

 the former being expressed or well expressed 

 in development), and 25 per cent, will bear 

 only the recessive character. This is called 

 the theory of the segregation of pure gametes, 

 and it is the corner-stone of Mendelism. A 

 scheme will make it clearer : 



Male Cells. Female Cells. 



U>DJ 1 pure dominant 



^^ V 2 impure dominants 



BE 



KB) 1 pure recessive 



