SEXUAL AND MENDELIAN REPRODUCTION 189 



recessive. Every individual is in fact an 'impure dominant.' 

 The non-sexual characters, on the other hand, are all patent. 

 When the breeding is intra-varietal, as it normally is in nature, 

 mating individuals (apart from their sexual characters) differ, 

 with rare exceptions, only to the extent of fluctuations, the in- 

 heritance of which, admittedly, is blended. The exceptions are 

 mutations which, like the sexual characters, tend not to blend, but 

 to become patent or latent in descendants. We saw also that 

 when natural varieties, which have arisen through the selection of 

 fluctuations, cross, they tend to blend their characters ; but when 

 artificial varieties, which have arisen through the selection of muta- 

 tions, cross, the reproduction of the characters in which they differ 

 tends to be alternative. Evidently, then, the mode in which muta- 

 tions are reproduced by offspring and descendants presents a close 

 parallel to the mode in which the sexual characters are reproduced. 

 312. The parallelism between the two sets of phenomena, the 

 sexual and the Mendelian, grows clearer the more we study it. 

 We have already noted many of the likenesses, but it will be useful 

 to bring them together, (a) Of necessity some of the sexual char- 

 acters, especially the primary characters, are developed in distinct 

 sets a male set and a female set which are kept separate by 

 selection. Most of the Mendelian characters, on the other hand, 

 are developed indifferently by both sexes. But many characters 

 the functions of which are sexual (in that they serve as attractions), 

 though not exclusively sexual, are reproduced in the latter way ; 

 for example, colour of eyes, and, to some extent, colour of skin and 

 hair in the intra-varietal matings of man, and colour of plumage 

 and hair in the inter-varietal matings of many birds and mammals. 

 (b) Sometimes characters peculiar to one sex are transferred to the 

 other, as in the cases of the bony cranial protuberance of the Polish 

 fowl 1 and the plumage of the Sebright bantam. Some Mendelian 

 characters are transmitted sexually, as colour-blindness and haemo- 

 philia in man. They would be termed sexual, not Mendelian, were 

 they more common. Deaf-mutism appears to furnish a further 

 connecting link, for the condition tends to affect the males in some 

 families and the females in others. 2 Moreover, the dominance and 

 recessiveness, the patency and latency of Mendelian characters 

 is sometimes affected by the sex of the parents, the offspring of 

 reciprocal crosses being unlike. 3 



1 Animals and Plants, vol. i., p. 269. 



8 C. J. Bond, British Medical Journal, Oct. 28th 1905. 



3 See 264; see also Bateson, Mendel's Laws of Heredity, chapter x. 



