DIVERGENT EVOLUTION THROUGH SEGREGATION. 333 
segregation, which is in an important degree positive. But even pre- 
potential segregation, when produced by mutual incompatibility be- 
tween a few individuals and a numerous parent stock, depends for its 
continuance and development on local, germinal, or floral segregation, 
partially securing the intergeneration of a few that are mutually com- 
patible. On the one hand, impregnational segregation depends on 
some degree of local, germinal, or floral segregation which is a constant 
feature in most species; but, on the other hand, not only do these initial 
forms of positive segregation fail of producing any permanent diver- 
gence till associated with impregnational segregation, but the more 
effective forms of positive segregation, such as industrial, chronal, fer- 
tilizational, sexual, and social segregation, often depend on impregna- 
tional segregation, inasmuch as the divergence of endowments which 
produces these depends on impregnational segregation. Moreover, in 
all such cases, increasing degrees of diversity in the forms of adapta- 
tion, and consequently of diversity in the forms of natural selection, 
must also depend upon these negative factors, which in their turn de- 
pend on the weak, initial forms of postive segregation. 
Divergent evolution always depends on some degree of positive 
segregation, but not always on negative segregation. Under a rigor- 
ous condition of the former (as for example complete geographical 
segregation), considerable divergence may result without any sexual 
incompatibility. Darwin has shown, by careful experiments, that inte- 
grate vigor and fecundity is the relation in which the varieties of one 
species usually stand to each other. This fact does not however prove 
that the more strongly divergent forms, called species, which are pre- 
vented from coalescing by segregate vigor and fecundity, did not 
acquire some degree of this latter character before any permanent 
divergence of form was acquired. Their having acquired this segre- 
gating characteristic may be the very reason why their forms are now 
so decidedly different, for without it they would have been swallowed 
up by the incoming waves of inter-generation. Again, we must remem- 
ber that forms only moderately divergent are habitually classed as dif- 
ferent 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 sufli- 
ciently explain why the varieties of one species are so seldom reported 
as mutually infertile. Notwithstanding this, the experiments of Gart- 
ner and of Darwin, already referred to at length, seem to show that 
segregate fecundity and vigor may arise between varieties that spring 
from one stock. In view of these cases, we must belieye that in the 
formation of some—if not many—species, the decisive event with which 
permanent divergence of allied forms commences is the intervent ion of 
segregate fecundity or vigor between these forms. Positive segrega- 
tion, in the form of local, germinal, or floral segregation producing only 
transitory divergencies, always one between the portions of a species 
