castle: role of selection in evolution 385 



Finally, as evidence that even in self-fertilized plants the pure 

 line principle may be inapplicable because of the existence of 

 genes which are plastic, let me cite a very extensive and care- 

 fully executed piece of work on garden peas done by Hoshino. 

 He studied the behavior of flowering time, and showed that its 

 inheritance involves a Mendelian gene coupled with flower 

 color (white or red). The inheritance of flowering time is inter- 

 mediate, but Fi is closer to the late than to the early parent in 

 this character. Segregation is imperfect in F 2 with a range 

 practically all the way from the early to the late parent, but not 

 transgressing this range. F 3 and F 4 families from self-fertilized 

 parents are in many cases quite variable but others are no more 

 variable than the pure parental varieties and so may be treated 

 as practically "constant." A study of the average flowering 

 time of each of the 230 "constant" F 4 families shows that these 

 fall into three main groups, some falling into a modified early 

 group, not quite so early as the early parent, others falling into a 

 modified late group, not quite so late as the original late parent, 

 but most of all falling into an intermediate group occupying the 

 region midway between the parent varieties in flowering time. 

 Considered all together, the F 4 families "constant" for flowering 

 time form an almost uninterrupted series of conditions con- 

 necting the respective parental conditions seen in the early 

 flowering and in the late flowering race. 



These observations show the existence of a gene for flowering 

 time in peas which is decidedly plastic. That a gene actually 

 exists is shown by its coupling with flower color. That it is 

 plastic is shown by the fact that it emerges from the cross nearly 

 always in a modified form. When the possibility of modifica- 

 tion has been continued as long as the F 4 generation, the majority 

 of the "constant" families are found in the intermediate or middle 

 group. The plasticity is here shown in a tendency of the con- 

 trasted genes to blend into one of intermediate character. It 

 is also shown in data given by Hoshino as to flowering time in 

 parent individuals and their offspring in the late flowering variety. 

 Although this variety is treated by Hoshino as a "pure line," 

 it is evident that within this line itself the later flowering in- 



