134 



EVOLUTION 



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 num- 

 bers. Now if each of the hybrids of the Fj 

 generation produces in both sexes 50 per 

 cent, of its germ-cells bearing the dominant 

 character and 50 per cent, bearing the re- 

 cessive 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: — 



I^Iale Cells. 



Female Cells. 



©••• 



(r^ ' ' ' (rr\ 



1 pure dominant 



2 impure dominants 



1 pure recessive 



