214 THE PHYSIOLOGY OF REPRODUCTION 



of va by spermatozoa of the same species. If such ova are present 

 the spermatozoa tend to conjugate with them rather than with ova 

 belonging to a different but closely allied species. It would appear, 

 therefore, that the spermatozoa exhibit an elective affinity for ova 

 belonging to the same species as themselves. This has been shown 

 especially in hybridisation experiments between brill and turbot. 1 



That assortative mating amongst gametes occurs generally as the 

 result of a preferential tendency possessed by them towards conjugating 

 with other gametes bearing similar characters to 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, 2 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' 3 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, 



1 M'Intosh and Masterman, Life History and Development of the Food Fithes, 

 and articles in the /!>/( f tl<>' *<-tti*h Fishery Boards, 9th Rep., Pt. III., 

 10th Rep., Pt. III., and 13th Rep., Pt. III. 



* Romanes, />///// <t,,<l .Iff,-,- M//-M-/, vol. iii., London, 1897. See also 

 h.irwin, Animal* anil /'/>n,(x, London, 1875, and Cross- and Xelf- Fertilization in 

 /'fa,,/*, [ndon, 1876. 



3 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 

 -i|iially, one may be "prepotent" over the other. (Cf. the Mendelian term 

 "dominant, 1 ' which has a more precise signification ; see p. 199.) 



