164 APPENDIX I—DIVERGENT EVOLUTION. 
regation, can do nothing toward bringing creatures together accord- 
ing to their compatibilities. The forms of segregation that place or 
draw together creatures of like innate characters I call forms of post- 
tive segregation. 
Of each form of segregation which we have up to this point con- 
sidered, 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 sim- 
ply on the original capacity for intergenerating 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 complete lack of capacity for fruitful 
crossing is the cause that prevents the production of mongrels, we find 
a dependence of a very different kind; for to insure the propagation of 
the different groups it is not enough that the general opportunity for 
the members of the species to meet and consort remains unimpaired. 
There must be some additional segregating influence bringing the 
members together in groups corresponding to their segregate capacity, 
or they will fail of being propagated. 
The form of impregnational segregation which I call prepotential 
segregation is due to the prepotency of the pollen of a species or variety 
on the stigma of the same species or variety, and complete potential 
segregation is due to the potency of the pollen of the same species, with 
the complete impotence 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, aided by the free dis- 
tribution of the pollen by the wind, 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 dicecious 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, the 
probability is that they will fail of being segregated through failing 
to receive any of the prepotent pollen. Itis thus apparent that when 
the mutually prepotent form is represented by comparatively few indi- 
viduals, 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 
and are flowering at the same time, their separate propagation is 
