170 APPENDIX I—DIVERGENT EVOLUTION. 
former. This form of segregation is evidently one of the important 
causes preventing the free crossing of different species of plants. It 
probably has but little influence on terrestrial animals; but how far 
it is the cause of segregation among aquatic animals is a question of no 
small interest, concerning which I have but small means for judging. 
I have, however, no hesitation in predicting that, unless we make the 
presence of this segregative quality the occasion for insisting that the 
forms so affected belong to different species, we shall find that amongst 
plants the varieties of the same species are often more or less separated 
from each other in this way. Ido not know of any experiments that 
have been directed toward the determining of this point; but on the 
general principle that race distinctions are the initial forms under 
which specific differences present themselves, I can have no doubt 
that feeble prepotence precedes that which is more pronounced, and 
that part of this divergence in many cases takes place, while the diver- 
gent branches may be properly classed as varieties. Another reason 
for believing that prepotential segregation will be found on further 
investigation to existin some cases between varieties is the constancy 
with which, in the case of species, this character is associated with 
segregate fecundity and segregate vigor, which we know are sometimes 
characteristics of varieties in their relation to each other. 
17,18. Segregate Fecundity and Segregate Vigor. 
By segregate fecundity I mean neither segregation produced by 
fecundity nor fecundity produced by segregation, but the relation in 
which species or varieties stand to each other when intergeneration 
of members of the same species or variety results in higher fertility 
than the crossing of different species or varieties. In like manner 
segregate vigor is the relation in which species or varieties stand to 
each other when the intergeneration of members of the same species 
or variety produces offspring more vigorous than those produced by 
crossing with other species or varieties. Integrate fecundity and 
integrate vigor are the terms by which I indicate the relation to each 
other of forms in which the highest fertility and vigor are produced by 
crossing, and not by independent generation. 
19. Segregate Adaptation.* 
Segregate adaptation is the relation in which species or varieties 
stand to each other when the intergeneration of individuals of the 
same species or variety produces offspring better adapted than the 

* This and the following paragraph were not in the paper as first published, 
though the advantage of escape from severe competition with members of the 
same species was set forth in the paragraph entitled ‘‘ Competitive disruption.” 
