96 THE EVOLUTION OF MAN 



females egg cells. These are the primary sexual characteris- 

 tics. But the sex of the great majority of higher animals can 

 also be determined by many features other than those of the 

 kind of reproductive cell discharged. Thus in man the male 

 as compared with the female is usually larger in stature and 

 more robust, has hair upon the face, a deeper voice, and many 

 other features all of which go to mark the sex as clearly and as 

 indubitably as the primary sexual character itself. These more 

 superficial and obvious distinctions are spoken of collectively 

 as the secondary sexual characters and serve, as has just been 

 intimated, with almost perfect certainty as an indication of sex. 

 If, however, there is any ambiguity in this respect, the final 

 resort in a decision is always to the primary sexual characters, 

 the kind of reproductive cells produced by the given individual. 



For a long time much interest has been expressed as to the 

 way in which the secondary sexual characters arise and it is 

 from this standpoint that hormones have come into much 

 prominence. Very remarkable experiments in this direction 

 have been recently carried out by Steinach on guinea pigs and 

 rats. The original object of this line of work was to ascertain 

 the influence of the sexual glands on the secondary sexual char- 

 acters, but the outcome of it was the discovery of a means of 

 influencing in a most profound way the sexual life of an 

 individual, in fact, so far as secondary sexual characters are 

 concerned, of converting a male into a female and vice versa. 



Steinach removed the male reproductive glands, the testes, 

 from young guinea pigs and, after they had recovered from 

 this operation, he implanted in their bodies by means of a 

 second operation the female gland or ovary from a young 

 female. The animal destined by nature to be a male but thus 

 harboring within its body the opposite sex gland was allowed 

 to mature, whereat a most remarkable transformation was 

 observed. It developed the bodily proportions of the female 

 in skeleton, hair coat, and other particulars, its mammary 



