206 THE PHYSIOLOGY OF REPRODUCTION 



their own, and that the comparative scarcity of hybrids in a 

 state of nature is very largely the result of this selective action, 

 are facts with which many of the older naturalists were familiar. 

 With reference to the various species of plants belonging to the 

 family Composite, Darwin wrote as follows : 



' There can be no doubt that if the pollen of all these species 

 could be simultaneously or successively placed on the stigma of 

 any one species, this would elect with unerring certainty its own 

 pollen. This elective capacity is all the more wonderful as it 

 must have been acquired since the many species of this great 

 group of plants branched off from a common progenitor." 



Romanes, 1 who quotes this passage, remarks that " 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 ' 2 of the pollen belonging to the same variety or 

 species over the pollen of another 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 fertilise 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 

 fertilising 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." 



It would appear, however, that when the aggregate vitality 



1 Romanes, Darwin and After Darwin, vol. iii., London, 1897. See also 

 Darwin, 1 n'mmlx and Plants, London, 1875, and Cross- and Self-Fertilisation 

 in Plants, London, 1876. 



a The term "Prepotency" is here used in a different sense to that in 

 which it is usually employed by zoologists, according to whom it means the 

 greater capacity of one parent, as compared with the other, to transmit its 

 characters to its offspring; thus, instead of both parents transmitting their 

 characters equally, one may be " prepotent " over the other. (Cf. the 

 Mendelian term " dominant," which has a more precise signification ; see 

 p. 194.) 



