442 Tricotylous Races. 



which is also present, for hardly ever is the intermediate 

 race found pure hy itself. As far as I know, at least, 

 there is no species of plants which has so much as half 

 of its seedlings showing three cotyledons, without being 

 selected. If we study the process of separation by the 

 statistical method, we find that two curves, a half curve 

 with an apex at 0, and a bilateral curve with an apex at 

 50 — 55%, can be distinguished from one another. Some- 

 times in one or other of the transitional generations 

 both curves can be more or less clearly seen side by side, 

 constituting a so-called dimorphic curve. I have fre- 

 quently observed this in these experiments, and have, in 

 some cases referred to it. 



Dimorphic curves of this kind are best obtained in the 

 transitional generations by planting out dicotylous as well 

 as tricotylous seedlings ; for, as was mentioned before, 

 it is very likely that many dicotyls will belong to the 

 half race and most of the tricotyls to the intermediate 

 race. But if the latter is once isolated, all the individuals 

 belong to it, independently of the question whether they 

 have tw^o or three or cleft cotyledons. In this stage a 

 dimorphism of this kind is no longer to be expected, un- 

 less selection is continued in two different directions.-^ 



As an instance of this let me cite the case of my cul- 

 ture of Mercnrialis annua in 1895 (see the table on page 

 439), inasmuch as this species, being dioecious, would 

 be expected to exhibit a levelling of the differential char- 

 acters. The plant with a value of 55% in 1894 could 

 obviously have been partly cross-fertilized. Its offspring 

 had in their seeds the following hereditarv values : 



Witli regard to this question compare the analogous experi- 

 ments witli H ^liantluis anmius syncotylcus (II, §ii). 



