STRUCTURAL AND POTENTIAL SEGREGATION. 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 coérdinations 
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 codrdinated 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.| 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 coérdination of the sexual elements enabling 
them to coalesce is maintained. 
Potential isolation occurs in the two forms, prepotential rsolation 
and complete potential isolation. Complete potential isolation exists 
* See Plant Structures, by John M. Coulter; Appleton & Co., 1900; pp. 176, 177. 
+ See Plant Relations, by John M. Coulter, pp. 129, 130. 
