STRUCTURAL AND POT^NTIAl, SEGRBGATION. 89 



cessive steps by which impregnation is reached. This last clause is 

 added, for we observe that in the language of recent botanists the 

 stamens and pistils of plants are not sexual organs; and "pollen 

 grains are asexual spores."* This, however, does not change the fact 

 that in order to secure fertilization the pollen grain, after reaching the 

 stigma, must be able to send out a pollen tube long enough and pene- 

 trating enough to descend through the length of the pistil to the center 

 of the ovule, through the nucellus and embryo-sac. The coordinations 

 required for securing these and many other steps in the process of 

 fecundation are maintained by impregnational selection, and so far 

 as they depend on the form and structure of organs we may call the 

 process "structural interselection." The propagation of every sex- 

 ually reproducing plant and animal must depend on such coordi- 

 nations. 



Structural isolation arises when local varieties that have become so 

 far divergent in structure as to be incompatible are brought together 

 in the same district. Darwin suggested that difference in the length 

 of the pollen tubes and the pistils may be the cause preventing crosses 

 between certain species of plants. 



9. The Potential Form of Selection and Isolation. 



Potential selection.— There are characters more fundamental than 

 form and structure that must be coordinated in order to secure the 

 fertilization of the ova that produce the next generation. The pollen 

 of one species of plants is usually either partially or entirely ineffective 

 if it falls upon the stigma of another species, even though both species 

 are of the same genus. There are also certain species having two 

 kinds of stamens producing two kinds of pollen ; and the pollen from 

 the short stamens is said to be most effective upon the stigmas of the 

 short styles, and the pollen from the long stamens most effective upon 

 the stigmas of the long styles. As each flower produces either a long 

 style and short stamens or a short style and long stamens, the dis- 

 criminate prepotence just described insures cross-fertilization. f But 

 our present interest is in the fact that in pollen grains there are char- 

 acters of an obscure nature which are of great importance in insuring 

 the required potency. There is reason to believe that in every species 

 depending on sexual reproduction there must be more or less potential 

 selection, by which the coordination of the sexual elements enabling 

 them to coalesce is maintained. 



Potential isolation occurs in the two forms, prepotential isolation 

 and complete potential isolation. Complete potential isolation exists 



* See Plant Structures, by John M. Coulter; Appleton & Co., igcw; pp. 176, 177. 

 t See Plant Relations, by John M. Coulter, pp. 129, 130. 



