Tricotyls as Half Races and Intcniicdiatc Races. 355 



as sharply and unalterably separated as, in the first part 

 of this volume, we saw was the case in numerous in- 

 stances, and especially in the tive-leaved race of the red 

 clover. Nothing less than a mutation can effect the 

 transition between the two, but I have not yet had the 

 good fortune to observe such an occurrence. 



In these experiments therefore the differences be- 

 tween individuals to which attention has to be paid are 

 their hereditary values ; and whether they themselves 

 have two, or three or cleft cotyledons is a matter of sec- 

 ondary importance. In my cultures the selection of tri- 

 cotyls as seed-parents has been the general rule, since 

 this practice on the one hand increases the probability 

 of excluding specimens wnth a low^ hereditary capacity, 

 and on the other, of including those with the high ; but 

 the increase of this chance is only a small one, as the 

 frequent cultures I have made from atavists clearly show 

 (see below, § 6). 



For a half race the curve describing these values is 

 a half curve. The vast majority of individuals have 

 either nothing, or little else, but dicotylous oft'spring; 

 and the numbers of individuals with the larger numbers 

 of tricotylous offspring decrease rapidly (Fig. 66). These 

 curves may be improved in the same way as those of 

 other half races, viz., by the selection of individuals witli 

 the highest value, as we have seen in Raintnculus bitl- 

 bostis scimplemis (see § 23, of the first part of this 

 volume, p. 249). 



Curves describing tliese values in intermediate races 

 usually have their maximum ordinates at 50% ; they are, 

 however, liable to be much altered by selection and ex- 

 ternal conditions. Fig. 67 is a curve of this kind f<^r 

 Oenothera Jiirfella, whose apex is at about 65%. If 



