146 MENDEL'S LAWS 



characters which are common to both sexes and which may have 

 other functions besides sexual attraction, development tends to be 

 alternative. Thus the child of parents, one of whom has blue eyes 

 and the other hazel, usually has blue eyes or hazel eyes, never eyes 

 that are a blend between blue and hazel. So too, in a lesser degree, 

 children sometimes reproduce the colour and texture of hair of 

 one parent, and to a still lesser degree the colour of the skin. 



240. Obviously the alternative development of the sexual 

 characters is an adaptation, a product of evolution. It is a device 

 by which cross-fertilization is secured. Though evolution has 

 caused most of the sexual characters to hang together in sets, yet 

 the separation of male and female characters is not always strictly 

 preserved. Occasionally a character, usually distinctive of one 

 sex, is found in an individual of the opposite sex. Thus, a man 

 may possess well-developed female breasts, or a woman a full 

 beard. Sometimes the male and female characters may be found so 

 intermixed, as for example in the so-called human hermaphrodites, 1 

 that an individual of one sex may present every external appear- 

 ance of belonging to the other. Or the male and female characters, 

 instead of being inherited alternatively, may blend in varying 

 degrees as when a woman is partially bearded or has a mascu- 

 line pelvis. 



241. Selection, natural and sexual, however, must always have 

 tended to eliminate these abnormalities and so preserve the purity 

 of the sexual characters, their alternative development, and their 

 aggregation in distinct sets. Selection is, of course, more stringent 

 as regards the primary sexual characters, which are the more 

 essential part of the machinery of reproduction, than as regards 

 the secondary characters. Consequently blending or displacement 

 from the set in which they are normally found is rarer in the case 

 of the former than the latter. Thus men may have female breasts and 

 women beards, but never, as far as I am aware, have testes been 

 found associated with a uterus. By artificial selection man has 

 more than once succeeded in transferring secondary sexual char- 

 acters from one sex to the other. Thus formerly only the female 

 Polish fowl had the domed skull and cerebral hernia which now 

 characterizes both sexes, 2 and the male Sebright bantam has the 



1 A real hermaphrodite is an individual in whom both sets of sexual characters, 

 the male and the female, are patent; for example, snails and most flowering 

 plants. A ' so-called ' hermaphrodite is an individual in whom the male and 

 female characters have blended, or one in whom some male and some female 

 characters are patent. 



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



