REPRODUCTION 3 



paratively long periods pass during which generations of 

 individuals are produced merely by simple division. But 

 at recurring intervals the individuals composing the swarm 

 at the moment, pair off and conjugate. Each individual 

 pairs only once. Maupas, who first observed these facts, 

 also showed that if conjugation was prevented by artificial 

 means, the race degenerated very rapidly. 1 The nuclei first 

 showed signs of degeneration, and eventually all the ani- 

 mals died. When conjugation was allowed to take place, 

 however, a fresh period of simple divisions occurred. 

 Though it has been shown that conjugation may be de- 

 layed, apparently almost indefinitely, by artificial condi- 

 tions of temperature and food, still it is certain that under 

 natural conditions conjugation is necessary for the perpetua- 

 tion of the race in the case of many of the unicellular forms, 

 though in some, particularly among unicellular plants, it 

 seems certain that it never occurs. 



Although among the unicellular organisms it appears 

 that there are some which can continue to propagate indefi- 

 nitely by the simple division of pre-existing individuals, in 

 the higher forms of animals two individuals are necessary 

 for the production of a new individual. Indeed, in the 

 higher animals we may say that bi-parental reproduction 

 is universal, for the only exceptions have apparently been 

 modified by selection from forms which have been bi- 

 parental. 2 



Among the higher animals the universality of bi-parental 

 reproduction is very obvious. Among the higher plants, 

 however, the matter is a little more complicated. The 

 body of the plant itself is formed of an asexual generation 

 of cells which has no counterpart in the life-history of the 

 higher animals. The sexual elements are produced by 



1 Maupas, Arch. ZooL, Exp. 2, Se"rie VI., 1888 ; Ibid., S6rie VII., 1889 ; 

 Comptes Eendus, Paris, 1891. 



2 The apparent exception of parthenogenesis is really only a modification, 

 more or less easily explained, of the usual form of bi-parental reproduction. 

 For the sake of clearness it is necessary to leave the detailed explanation until 

 later. 



