THROUGH CUMULATIVE SEGREGATION. 
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 effectually as a higher degree ; but further 
reflection will show that the efficiency 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 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 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 be 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 
