THE AMEBIC AN NATURALIST [Vol. LII 



the division of labor made possible by secondary sexual 

 characters, using the term very generally and including 

 even such differences as those which separate the egg 

 and the sperm. It is not known just how these differences 

 arose or by what mechanism they are transmitted. The 

 greatest hope of reading the riddle lies in an investiga- 

 tion of hermaphroditic plants, for there are technical 

 difficulties which seem to preclude their solution in ani- 

 mals. For example, breaks in the linkage between sex- 

 linked characters occur only in the female in Drosophila, 

 and as the sex chromosome is double in the female, it 

 can not be determined whether the differentiation be- 

 tween male and female is due to the whole chromosome or 

 not. But this ignorance does not give reason for a denial 

 of the great advantage which sexes bearing different 

 characters hold over sexes alike in all characters except 

 the primary sex organs. 



The only glimpse of the truth we have on these matters 

 comes from recent work on the effect of secretions of the 

 sex organs on secondary sexual characters. The effect of 

 removing the sex organs and the result of transplanting 

 them to abnormal positions in the body have shown that 

 in vertebrates the secretions of these organs themselves 

 activate the production of the secondary sexual char- 

 acters. This does not seem to be the case in arthropods, 

 however, so one can not say that primary sexual differ- 

 entiation and secondary sexual differentiation is one and 

 the -ante thing. 



Finally there is a presumable advantage in gonocho- 

 ristic reproduction in having sex-linked characters. We 

 say presumable advantages, for all of the relationships 

 between sex and sex-linked characters are not clear. The 

 facts are these: One sex is always heterozygous for the 

 sex determiner and the factors linked with it. Now it 

 may very well be that there is a.n actual advantage in the 

 heterozygous condition, as we have seen above. But 

 should the so-called vigor of heterozygosis prove to be 

 only an expression of the meeting of dominant characters, 

 still a possible advantage accrues to this phenomenon be- 



