168 Life and Death, Heredity and Evolution 



resistant stocks resulting from the new combinations formed 

 in mating. There is ground for believing that in nature 

 this process occurs on a large scale. 



Looking back over what has been found out as to the 

 effects of mating, the general picture is as follows : It has 

 been shown that infusoria may live and multiply indefinitely 

 without conjugation (Woodruff, Enriques). It has been 

 shown that at intervals the old active macronucleus is re- 

 placed by a part of the reserve micronucleus. These things 

 demonstrate that the mating process (as distinguished from 

 the replacement process) is not necessary for continued life 

 and vigor. They appear to disprove any theory of sexuality 

 that maintains that there must for continued life be a 

 periodic reunion of two substances, male and female, which 

 inevitably become separated as a result of life and develop- 

 ment. Rejuvenescence is through the replacement of used 

 parts by unused ones, and this occurs without mating, al- 

 though it may occur at mating also. The distinctive con- 

 tribution of the mating itself is something else. 



Investigation shows that mating produces two very strik- 

 ing results: (1) It causes the offspring of the two individu- 

 als that have conjugated to become more alike; it produces 

 biparental inheritance. (2) It causes the different families 

 produced by different pairs to be hereditarily diverse in many 

 respects; and this even when all the parents come from a 

 single ancestor and are hereditarily alike. 



Do we find anything of this sort elsewhere in organisms? 

 Consideration brings to light the fact that this is precisely 

 what results from mating in higher organisms; we call the 

 detailed working out of these results Mendelian heredity. 

 In heredity in higher organisms, the offspring produced by 

 any pair resemble each other more than they do other in- 

 dividuals; they show biparental inheritance. Furthermore, 



