114 Life and Death, Heredity and Evolution 



interested in now is precisely the question as to the effect 

 of the mating as distinguished from the process of replace- 

 ment. A second answer to the question as to how rejuvenes- 

 cence is brought about holds that it is due primarily to this 

 union of two individuals or of two nuclei. In the minds of 

 many that hold it the grounds for this belief are undefined; 

 Maupas (1889, p. 486) in his great work on rejuvenescence, 

 in which he maintains this theory, avows distinctly that he 

 cannot see how the union of nuclei should produce re- 

 juvenescence, though he believes that it does. But there has 

 grown up a theory as to how the rejuvenescence is brought 

 about by union ; a theory that is rather generally held. This 

 theory depends upon the farther theory that there is a 

 fundamental difference between the male and female sexes, 

 and that this essential sex difference is always present in 

 mating, and is the basis of rejuvenescence. We know indeed 

 that in many organisms the differences between the sexes are 

 not mere superficial diversities, but that the two sexes differ 

 in every cell of their bodies, and the observable difference 

 lies precisely in the nuclei, which we know to be funda- 

 mentally important parts of the organism. In the com- 

 moner cases, including man, the nuclei of the female have one 

 more chromosome than have those of the male (Figure 31, 

 A and B) ; in other cases one chromosome of the female 

 differs from the corresponding one in the male (Figure 

 31, C and D). Such a visible structural difference neces- 

 sarily means further a diversity in the most fundamental 

 and intimate physiological processes of the two sexes; in 

 the chemical changes that determine the nature of life. 

 These chemical changes we observe to occur in the physio- 

 logical interaction between the nucleus and the rest of the 

 protoplasm. 



What are the fundamental physiological differences be- 



