Progressive, Retrogressive, Degressive Mutations. 573 
sionally manifested as an anomaly; as for instance the 
appearance of female flowers on male specimens of dioe- 
cious 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 by 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 question 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 qtiinquefoliate 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 
