252 Heredity. 



the female element of another; that is, we have true 

 sexual reproduction in its simplest form. 



Among the lower animals and most ])lants both sexes 

 arc united in the same individual, but the law of physio- 

 logical division of labor, the principle that an organ or 

 organism, like a machine, can do some one thing better 

 and with less expenditure of force when it is specially 

 adapted to this one thing than when it is generally 

 adapted for several functions, would lead to the preser- 

 Yation by natural selection of any variations in the di- 

 rection of a separation of the sexes, and we should there- 

 fore expect to find among the higher animals what we 

 actually do find — the restriction of the male function to 

 certain individuals, and the restriction of the female 

 function to others. From this time forward the male 

 is an organism specialized for the production of the vari- 

 able element in the reproductive process, and the female 

 an organism sjiecialized for the production of the con- 

 servative element. We soon meet with structural pecu- 

 liarities adapted to'aid and perfect the performance of 

 these respective functions; and the various organs, habits, 

 and instincts by which, among the higher animals, the 

 rearing of young is provided for, form one of the most 

 interesting chapters of natural science. On a priori 

 grounds we should expect a still greater specialization 

 to make its appearance. Since the male organism has 

 for its function the production of the variable reproduc- 

 tive element, and since variations which originate in a 

 male have their perpetuation especially provided for, it 

 would clearly be of advantage that the male organism 

 should acquire a peculiar tendency to vary, and any 

 steps in this direction would accordingl}^ be seized upon 

 by natural selection and perpetuated. The female or- 

 ganism, on the other hand, having for its function the 



