NORMAL FERTILIZATION 219 
Leia and Veluttna respectively, possess female gametes 
half of which can be shown to bear the tall and half 
the dwarf character. All the functional pollen grains 
of Leta 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, Cnothera lata 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, 
possibly by a definite substance or combination of sub- 
stances. The pair of parental factors for a particular 
character would combine on fertilization, and at the 
