’THROUGH CUMULATIVE SEGREGATION. 
239 
other forms of Segregation, which I group together as Positive 
Segregation. 
Of each form of Segregation which we have up to this point 
considered, the segregating cause has been one that distributes 
individuals of the same species in groups between which free 
intergeneration is checked ; while the propagation of the different 
groups depends simply on the original capacity for intergenera- 
ting common to all the members of the species. The intercrossing 
has been limited not by the capacity, but by the opportunity and 
inclination of the members. Coming now to cases in which the 
lack of capacity is the cause that checks the production of 
mongrels, we find a dependence of a very different kind ; for to 
ensure the propagation of the different groups it is not enough 
that the general opportunity for the members to meet and con- 
sort remains unimpaired. There must be some additional segre- 
gating influence bringing the members together in groups corre- 
sponding to their segregate capacity, or they will fail of being 
propagated. 
A partial exception must be made in the case of Potential 
and Prepotential Segregation, the latter being due to the pre- 
potency of the pollen of a species or variety on the stigma of the 
same species or variety, and the former to the complete impo- 
tence of the foreign pollen. When allied species of plants are 
promiscuously distributed over the same districts, and flowering 
at the same time, prepotency of this kind is one of the most 
direct and efficient causes of Segregate Breeding. The same 
must be true of varieties similarly distributed whenever this 
character begins to affect them. In the case, however, of 
dioecious plants and of plants whose ovules are incapable of being 
impregnated by pollen from the same plant, no single plant can 
propagate the species. If, therefore, the individuals so varying 
as to be prepotent with each other are very few and are evenly 
distributed amongst a vast number of the original form, they 
will fail of being segregated through failing to receive any of the 
prepotent pollen. It is thus apparent that when the mutually 
prepotent form is represented by comparatively few individuals, 
their propagation without crossing will depend on their being 
self-fertile and subject to Germinal or Floral Segregation, or on 
their being brought together by some other form of Positive 
Segregation. 
When a considerable number of species of plants are commingled 
19 * 
