90 Darwin, and after Darwin. 



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 

 betvyeen " 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 per- 

 ceptible 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 differentia- 

 tion between two varieties, and ending in an absolute 

 partitioning of two allied species— we have the only 



