THE FUNCTION OF SEX 



313. (c) The alternative development the alternative patency 

 and latency of sexual characters is more perfect than that of most 

 Mendelian characters, but this is only what might have been 

 expected ; for it has been secured by selection, just as the less 

 perfect, or rather the different alternation in aphides, and bees has 

 been similarly secured, (d) When Mendelian characters are crossed, 

 one of each pair is usually the dominant in the first hybrid genera- 

 tion. Thus colour is usually dominant over white in mammals 

 and flowers, whereas the contrary is the case in poultry. In the 

 case of sexual characters one or other alternative dominates in any 

 individual. But sometimes the offspring of a single pair are 

 wholly of one sex, a phenomenon not very rare in human families ; 

 whilst, on the other hand, in the case of some Mendelian alterna- 

 tives, both types tend to appear in the first hybrid generation for 

 example, normal and extra toes in poultry, (e) As might have 

 been expected also, the sexual characters show a lesser tendency to 

 blend than Mendelian characters ; but some degree of blending is 

 not uncommon in the former, as when women have masculine pelves 

 and partially bearded faces. (/) The crossing of varieties may 

 bring to light long dormant Mendelian characters for example, 

 wild-grey in mice and rabbits ; it may also reveal strictly sexual 

 characters which have long been latent ; for example, broodiness 

 and sexual peculiarities of plumage in fowls ; and sometimes the 

 lost sexual characters may reappear in connection with traits 

 properly belonging to the opposite sex. 1 (g) As Ewart 2 and 

 others have shown, the dominance of both sexual and non-sexual 

 alternative characters is capable of being influenced by the 

 environment. 



314. (k) If the characters of crossed varieties blend or are lost, 

 there is a very general tendency to sterility, as is the case when 

 natural varieties interbreed ; but if they alternate so that the char- 

 acters are unchanged, as when artificial varieties cross, fertility is 

 perfect. " We know no Mendelian case in which fertility is im- 

 paired." 3 We have here a very clear indication of the parallelism 

 between sexual and Mendelian characters, for under such conditions, 

 as in mules, sexual characters also tend to blend or be lost with con- 

 sequent sterility, (z) Mendelian characters which segregate in later 

 generations are sometimes blended in the first hybrid generation 

 for example, in the case of Andalusian fowls ; or there may be 

 particulate inheritance in the first generation and more complete 



i See 280. 2 See 132. 



3 First Report of Evolution Committee, p. 148. 



