Darwin is here speaking of elective affinity in its fully developed form, as absolute cross-sterility between fully differentiated species. But we meet with all lower degrees of cross-infertility--sometimes between "incipient species," or permanent varieties, and at other times between closely allied species. It is then known as "prepotency" of the pollen belonging to the same variety or species over the pollen of the other variety or species, when both sets of pollen are applied to the same stigma. Although in the absence of the prepotent pollen the less potent will fertilize the seed, yet, such is the appetency for the more appropriate pollen, that even if this be applied to the stigma some considerable time after the other, it will outstrip or overcome the other in fertilizing the ovules, and therefore produce the same result on the next generation as if it had been applied to the mother plant without any admixture of the less potent pollen, although in some cases such incipient degrees of cross-infertility are further shown by the number or quality of the seeds being fewer or inferior. Now, in different varieties and in different allied species, all degrees of such prepotency have been noticed by many observers, from the faintest perceptible amount up to complete impotency of the alien pollen--when, of course, there is absolute sterility between the two varieties or allied species. The inference is obvious. In this graduated scale of prepotency--beginning with an experimentally almost imperceptible amount of sexual differentiation between two varieties, and ending in an absolute partitioning of two allied species--we have the only remaining fact that is required to complete the case in favour of the present theory. We are here brought back to the very earliest stages of physiological differentiation or to the stages which lie behind Jordan's "Physiological Species"; and therefore, when taken in conjunction with his results, the phenomena of prepotency may be said to give us the complete and final demonstration of one continuous development, which, beginning in an almost imperceptible amount of cross-infertility, ends in absolute cross-sterility. The "elective capacity" to which Darwin alludes as having been "acquired" by all the species of Compositae since they "branched off from a common progenitor," is thus seen among innumerable other species actually in process of acquisition; and so we can perfectly well understand, what is otherwise unintelligible, that closely allied species of plants occur, in ninety-five per cent. of cases, intimately associated on common areas, while exhibiting towards one another the character of mutual sterility.