NATURAL SELECTION 133 



belong to the same groiq) — their gerni-j^lasm cannot 

 be insjDected. Hence selection of bi2)arental forms is 

 a precarious process, now going forward, now back- 

 wards, now standing still. In time, however, the pro- 

 cess forward is almost certain to take place j)rovided 

 the selection is from a heterogeneous population. 

 Johannsen's work was simplified because he started 

 with pure lines. In fact were this not the case his 

 work would not have been essentiallv different from 

 that of any other selection experiment. 



It has since been j)ointed out by Jennings and by 

 Pearl that a race that reproduces by self-fertilization, 

 as does this bean, automatically becomes pure in all 

 of the factors that make up its germ-material. Since 

 self-fertilization is the normal process in this bean 

 the purity of the germ-plasm of each line already 

 existed when Johannsen began to experiment. 



Ge7ietic Vaiiahility 



In addition to the variability due to external fac- 

 tors acting on the individual during its development 

 there are also differences in the germ-materials — 

 genetic factors — that are known to produce slight 

 differences in the extent to which some particular 

 part or character develops. Inasmuch as some of 

 these minor or modifying genetic factors produce 

 their results only in the presence of the chief char- 

 acter, thev may be concealed and only manifest their 

 presence when the chief character develops. The 



