SEX-CHARACTERS IN EVOLUTION 119 



SEXUAL DIMORPHISM 



It is obvious from the above facts that however 

 interesting and important sex-linked heredity may be, 

 it is not the same thing as the heredity of secondary 

 sexual characters, and does not in the least explain 

 sexual dimorphism. In the first place, the term 

 sex-linked does not mean occurring always ex- 

 clusively in one sex, but the direct contrary trans- 

 mitted by one sex to the opposite sex and in the 

 second place there is no suggestion that the develop- 

 ment of the character is dependent in any way on 

 the presence or function of the gonad. The problem 

 I am proposing to consider is what light the facts 

 throw on the origin of the secondary sexual characters 

 in evolution. In endeavouring to answer this ques- 

 tion there are only two alternatives: either the 

 characters are blastogenic that is, they arise from 

 some change in the gametocytes occurring some- 

 where in the succession of cell-divisions of these 

 cells or they arise in the soma and are impressed 

 on the gametocytes by the influence of the soma 

 within which these gametocytes are contained 

 that is to say, they are somatogenic. That characters 

 do originate by the first of these processes may be 

 considered to be proved by recent researches, and 

 such characters are called mutations. There can be 

 little doubt that the so-called sex-linked characters, 

 of which examples have been given above, have 

 originated in this way, and that their relation to sex 

 is part of the mutation. According to T. H. Morgan, 

 it is simply due to the fact that the determinants for 

 such characters are situated in the sex-chromosome. 

 Morgan, however, also states that a case of true 



