EVOLUTION 207 



changed would probably lose in the race with more adapt- 

 able cross-fertilized forms. Their handicap is their lack 

 of chances for progress. 



A secondary advantage of sexual reproduction is the 

 division of labor made possible by secondary sexual char- 

 acters, 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 great- 

 est hope of reading the riddle lies in an investigation of 

 hermaphroditic plants, for there are technical difficulties 

 which till now have prevented its solution in animals. For 

 example, breaks in the linkage between sex-linked char- 

 acters occur only in the female in Drosophila, and as the 

 sex chromosome is double in the female, it cannot be de- 

 termined whether the differentiation between 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 the matter 

 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 cannot say that primary sexual differ- 

 entiation and secondary sexual differentiation are one and 

 the same thing. Nevertheless, the generalization is not 



