70 REPRODUCTION 



certain that the development of the sex organs and of the 

 cells within these is the chief cause of the differences we 

 see between male and female animals. For example, in 

 the very early development of an individual it is usually 

 impossible to tell which sex an animal is going to be. It 

 is only as the testes and the ovaries develop that the 

 external signs of sex are shown. If we take a male 

 chicken in early life and remove its testes, it will fail to 

 develop spurs and some other externa^l signs of its 

 maleness. Much the same thing is true if the ovaries of 

 a young female chicken are removed. It is much more 

 difficult to see any difference between a male and a 

 female thus deprived of the internal sex organs than 

 between the normal, natural sexes. This shows that it is 

 the sex cells that determine the nature of the parent body 

 rather than that the nature of the parent body determines 

 the sex cells. 



8. The Advantage of these Differences. We know 

 that the developing testes and sperms put into the blood 

 materials that stimulate the growth of the male characters 



even in such remote parts as the vocal organs, muscles, 

 and so forth. It is the presence of these substances that 

 stimulates the growth of muscles and nervous system. 

 If these substances are absent because of removal, or 

 because of abuse of the developing sex organs, the 

 physical and mental qualities do not develop normally. 



You may wonder what value these differences between 

 males and females have. Or you may take for granted 

 that the sexes should be different and ask no questions 

 about it. Why should the males differ in so many ways 

 from the females? Is there any advantage to the species 

 in having them different? 



We have seen that sperms and eggs stimulate the 

 individuals producing them very differently. There is, 

 however, another side to the matter. The task of the 

 male, the work done by him, and the service rendered 



