NORMAL FERTILIZATION 219 



half of which can be shown to bear the tall and half 

 the dwarf character. All the functional pollen grains 

 of Lata bear the tall character, and all the functional 

 pollen grains of Velutina bear the dwarf character. 

 De Vries does not offer the obvious suggestion that 

 half of the total number of pollen grains in each case 

 are impotent.* If this suggestion should prove to 

 represent the truth, (Enothera Iceta would afford a 

 case of a hybrid which breeds true in spite of the fact 

 that typical segregation is taking place among its 

 germ-cells. In the light of this discovery it is clear 

 that non-segregation cannot properly be asserted in 

 any given case until the hybrid has been crossed with 

 each of the parent forms on a considerable scale. 



We may now turn for a brief space to some of the 

 cases in which we have as yet no certain knowledge of 

 the manner in which inheritance proceeds. 



The most obvious extension of Mendel's law to 

 processes where it cannot be directly shown to hold 

 good is to suppose that the same rule applies to cases 

 of normal fertilization as to hybrid fertilizations. We 

 should then picture the former process as taking 

 place in somewhat the following way. Every visible 

 character of the individual which can be separately 

 distinguished, and which on cross-breeding would be 

 inherited on ordinary Mendelian lines, must be repre- 

 sented in the gametes by a definite factor of some kind, 



* [In an analogous case in Drosophila it seems certain that the 

 explanation is of this kind. See H. J. Muller * Genetic Variability, 

 Twin Hybrids and Constant Hybrids, in a case of balanced lethal 

 factors.' GtneticS) vol. Hi., 1918, p. 422-499.] 



