SEXUAL INCOMPATIBILITY. 173 
segregation of a rigorous form (as, for example, complete geograph- 
ical segregation), considerable divergence may result without any 
sexual incompatibility. Darwin has shown, by careful experiments, 
‘that integrate vigor and fecundity is the relation in which the varieties 
of one species often stand to each other. This fact does not, how- 
ever, prove that the more strongly divergent forms, called species, 
which are prevented from coalescing by segregate vigor and fecundity, 
did not acquire some degree of this latter character before any perma- 
nent divergence of form was acquired. Their having acquired this 
segregating characteristic may be the very reason why their forms are 
now so decidedly different, for without it they would have been swal- 
lowed up by the incoming waves of intergeneration. Again, we must 
remember that forms only moderately divergent are habitually 
classed as different species if they are separated by segregate vigor 
and fecundity (that is, by some degree of mutual sterility), unless 
observation shows that they are of common descent. These two 
considerations sufficiently explain why the varieties of one species 
are so seldom reported as mutually infertile. Notwithstanding this, 
the experiments of Gartner and of Darwin seem to show that seg- 
regate fecundity and vigor may arise between varieties that spring 
from one stock. In view of these cases we must believe that in 
the formation of some, if not many species, the decisive event 
with which permanent divergence of allied forms commences is the 
intervention of segregate fecundity or vigor between these forms. 
Positive segregation, in the form of local, germinal, or floral segrega- 
tion, producing only transitory divergences, always exists between the 
portions of a species that has many members; but as it does not 
directly produce the negative segregation which is, in such cases, the 
necessary antecedent of permanent divergence, we can not, in accord- 
ance with the usage of language, call it the cause of the permanent 
divergence. Moreover, though it may be in accordance with ordinary 
language to call the negative segregation, which is the immediate 
antecedent of the permanent divergence the cause of the same, it will 
be more correct to call the coincidence of the negative and positive 
segregations the cause, and still more accurate to say that the whole 
range of vital activities (when subjected to the limitations of any 
sexual incompatibility that corresponds in the groups it separates to 
some previous but ineffectual local, germinal, or floral segregation) 
will produce permanent divergence. 
In many cases not only is the entrance of impregnational segrega- 
tion the cause of the commencement of permanent divergence, but its 
continuance is the cause of the continuance of the divergence. The 
