584 Species According to the Theory of Mutation. 
ous forms or elementary species and derived forms or 
true varieties. 
As we concluded in 7, Part I, p. 65, the origin of 
elementary species is due to the formation of new ele- 
mentary characters, i. e., to their actual numerical in- 
crease. True varieties differ from the species to which 
they belong by the latency of certain characters, which 
may either be active as in the type of the species, or latent 
as in the variety, or which may occur in a latent or semi- 
latent condition in the former, and become active or semi- 
active in the production of the variety. In other words, 
we may say that elementary species arise by progressive 
mutations, but derivative varieties by retrogressive and 
degressive ones (p. 71). 
If we now compare the principles derived from the 
study of hybrids with these conclusions, we see that the 
two main types of hybrids are in essential agreement 
with these two systematic groups. Mendelian hybrids 
correspond to retrogressive and degressive specific dif- 
ferentiation, and consequently to true varieties; uni- 
sexual hybridizations correspond to progressive specific 
differentiation and consequently to elementary species. 
There can hardly be any misconception as to the sig- 
nification of this important conclusion. But it only indi- 
cates the principle and not its application to particular 
cases; and, as a matter of fact, it is only another form 
of the generalization enunciated above relating to the 
fertility of crosses and hybrids. For Mendelian crosses 
have as a rule the same fertility as the pure parent forms, 
and fertility does not diminish in the subsequent hybrid 
generations. In uni-sexual crosses, however, fertility di- 
minishes and it does so in proportion as the relation be- 
tween the two forms crossed becomes more remote. 
