314 DIVERGENT EVOLUTION THROUGH SEGREGATION. 
forms are able to maintain vigor and secure fair sustentation, the pro- 
cess continues and the separation becomes more pronounced. Of this — 
form of the law of cumulative segregation we may say, that as the 
descendants of the best titted necessarily generate with each other and 
produce those still better fitted, so the descendants of those possessing 
the most segregative endowments necessarily generate with each other 
and produce those that are still more segregate. 
It may at first appear that a slight degree of pre-potence will prevent 
crossing as effectually as a higher degree; but further reflection will 
show that the efficiency of the prevention will vary in direct propor- 
tion with the length of time over which the pre-potent pollen is able to 
show its pre-potence, and this will allow of innumerable grades. If, in 
the case of certain individuals, the pre-potency 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 in 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 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 segrega- 
tion among aquatic animals is a question of no small interest, concern- 
ing 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 vari- 
eties 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 prin- 
ciple 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 pre-potence precedes that 
which is more pronounced, and that part of this divergence in many 
cases takes place, while the divergent branches may be properly classed 
as varieties. Another reason for believing that pre-potential segrega- 
tion 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. The importance of these latter principles when 
occurring in connection with different forms of partial segregation will 
now be considered. 
17,18. Segregate fecundity and segregate vigor.—By segregate fecun- 
dity 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 the inter-generation of members of the 
