120 ORIGIN OF SOMATIC 



sexual dimorphism arose as a mutation in his 

 cultures of Drosophila, The character was eosin 

 colour in the eye instead of the red colour of the eye 

 in the original fly. In the female this was dark 

 eosin colour, in the male yellowish eosin. But this 

 case differs from the characters particularly under 

 consideration here in two points: (1) there is no 

 suggestion that it was adaptive, (2) or that it was 

 influenced by hormones from the gonads. 



No character whose development is dependent in 

 greater or less degree on the stimulation of some 

 substance derived from the gonads can have origin- 

 ated as a mutation, because the term mutation 

 means a new character which develops in the soma 

 as a result of the loss or gain of some factor or de- 

 terminant in the chromosomes. To say that certain 

 mutations consist of new factors which only cause 

 the development of characters in the soma when 

 the part of the soma concerned is stimulated by a 

 hormone, is a mere assertion unsupported at present 

 by any evidence. As an example of the way in 

 which Mendelians misunderstand the problem to be 

 considered, I may refer to Doncaster's book, The 

 Determination of Sex,^ in which he remarks : ' It 

 follows that the secondary sexual characters cannot 

 arise simply from the action of hormones ; they 

 must be due to differences in the tissues of the body, 

 and the activity of the ovary or testis must be re- 

 garded rather as a stimulus to their development 

 than as their source of origin.' This seems to imply 

 a serious misunderstanding of the idea of the action 

 of the hormones from the gonads and of hormones 

 in general. No one would suggest that the hormones 



1 Camb. Univ. Press, 1914, p. 99. 



