Process of Evolution 219 



acters. On the other hand there is little indication that 

 when an agent produces a direct effect on a part of the body, 

 this so changes the germ cells that in later generations they 

 produce bodies with the same alterations. 



All together we find then, that even independently of any 

 mating processes, diversity of stocks is being produced, but 

 most slowly and gradually. 



Next we turned to a study of reproduction from two par- 

 ents, and its relation to these general questions. We found 

 that while in many organisms (particularly the higher 

 ones), the two individuals or cells that mate are unlike, be- 

 longing to separate sexes, this seems not to be universal. 

 Mating is apparently often between like parts. This ap- 

 pears clearest in the final act of mating, the conjugation of 

 the chromosomes, for in these there is no indication of a 

 sex difference. But it seems to be true also in many cases 

 for the germ cells and for the individuals that mate. It is 

 certainly not clear that sex diversity is a general and fun- 

 damental requisite for mating ; rather does the contrary ap.- 

 pear true. 



Yet where sex diversity does occur, as in the higher or- 

 ganisms, it is manifest in the most fundamental features of 

 the organism. Every cell of the male, in many organisms, 

 differs from every cell of the female, and precisely in the 

 most fundamental features of the cell ; in the nuclei ; and in 

 the essential chemical operations in which the nuclei are in- 

 volved. But this seems to be a condition derived from the 

 simpler state where no such diversity exists, but in which 

 mating nevertheless occurs. 



In search for what is fundamental in mating and its re- 

 sults, we came upon theories that mating produces re- 

 juvenescence; that mating is a necessity for continued ex- 

 istence and multiplication; that without it vitality is lost; 



