THROrGH CUMULATIVE SEGBEGATION. 



245 



we may say that, as the descendants of the best fitted necessarily 

 generate with each other and produce those still better fitted, so 

 the descendants of those possessing the most segregative endow- 

 ments necessarily generate with each other and produce those 

 that are still more segregate. 



It may at first appear that a slight degree of prepotence will 

 prevent crossing as eff'ectually as a higher degree ; but further 

 reflection will show that the efiiciency of the prevention will vary 

 in direct proportion with the length of time over which the pre- 

 potent pollen is able to show its prepotence, and this will allow 

 of innumerable grades. If, in the case of certain individuals, 

 the prepotency is measured by about twenty minutes, while with 

 other individuals it enables the pollen of the same variety to 

 prevail, though reaching the stigma an hour after the pollen of 

 another variety has been applied, the difference iri the degree of 

 Segregation will be sufficient to make the persistence of the 

 latter much more probable than that of the former. This form 

 of Segregation is evidently one of the important causes prevent- 

 ing 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 

 physiological evolution is not usually abrupt, and 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 divergent branches 

 may bo properly classed as varieties. Another reason for believ- 

 ing that Prepotential Segregation will be found on further inves- 

 tigation 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 Vigour, which we know 

 are sometimes characteristics of varieties in their relation to each 

 other. The importance of these latter principles when occurring 



