IX] HEREDITY AND SEX 133 



sterile female birds of plumage resembling that of 

 the male ; and lastly by the occasional development 

 in one sex of characters which are normally rudi- 

 mentary in that sex, but are well developed in the 

 other, such as the breasts in man. If such characters 

 are not directly connected with reproduction, for 

 example the splendid plumage of some male birds or 

 the horns of deer, they are called secondary sexual 

 characters, but there is no sharp distinction between 

 these and accessory sexual structures such as the egg- 

 laying apparatus of female insects. 



It was said above that the appearance of sexual 

 characters is to some extent dependent on environ- 

 ment, but in fact the only changes which can modify 

 their development are those Avhich aifect the essential 

 reproductive organs, the ovaries and testes. When 

 these are removed or suppressed, the sexual characters 

 may be modified ; their appearance indeed depends 

 in many cases upon the presence of functional sexual 

 organs, and frequently they do not appear until the 

 sexual organs are mature. Now it has been seen 

 above that certain characters are dominant in one sex 

 and recessive in the other, and the most typical case 

 of this is in an actual secondary sexual character — 

 horns of sheep, for when horns are present they are 

 always better developed in the male. Their 'recessive' 

 condition in a ewe produced by crossing a horned by 

 a hornless breed must probably be ascribed to a 



