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. I do 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 exist in 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." 



