THEOUGH CtJMULATlVE 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* 



