Progressive, Retrogressive, Degressive Mutations. 573 



sionally manifested as an anomaly; as for instance the 

 appearance of female flowers on male specimens of cli(L"- 

 cions species (Fig. 134), or of leaves on normally leaf- 

 less inflorescences (Fig. 133). 



Two races, which only differ in the latency or activity 

 of a single factor, therefore possess the same number of 

 elementary units in their internal organization. Ob- 

 viously the relation between them is different from that 

 between two races, of which one has arisen from the 

 other bv the formation of a new factor; in which case 

 there is a difference of one, in the number of units, be- 

 tween the two. 



But before we examine this relation more closely we 

 must face the Cjuestion whether the active and the in- 

 active states are the only ones in which an internal factor 

 can occur. Theoretically this is obviously not necessarily 

 the case, for we can easily imagine various degrees of 

 activity between the two extremes, and as a matter of 

 fact, experience shows that these intermediate stages do 

 actually occur. We have described them above (p. 20) 

 as semi-latent ; and have given the name of middle races 

 to those which possess such semi-latent characters. Of 

 these there are two types, which we frequently meet both 

 in nature and in our cultures, half races and intermediate 

 races, or eversporting varieties. In both of them the 

 semi-latent quality is associated in such a way with some 

 active character that the two cannot be manifested at the 

 same time. They exclude one another, if we may so ex- 

 press it, and so constitute a mutually vicarious pair. Tri- 

 foliate and quinciuefoliate leaves of clover, tricotylous 

 seedlings or split cotyledons and dicotylous ones, normal 

 and peloric flowers, cylindrical and fasciated stems, ordi- 

 nary and petaloid stamens, constitute such pairs. The 



