FERTILISATION 29 



It is evident, however, that this phenomenon has been 

 produced in the course of evolution as a modification of 

 bi-parental reproduction. Sex is present, but has ceased 

 to be functional. The evidence at our disposal makes it 

 clear that in these multicellular organisms bi-parental re- 

 production existed at some time in their previous history, 

 but that it has been wholly or partially eliminated in some 

 cases, and that here only one sex is necessary. 



This is the general course of events with regard to fer- 

 tilisation among the multicellular animals and plants. Some- 

 thing similar, however, has been shown to occur among 

 unicellular organisms, both animals and plants. This 

 phenomenon was first described by Maupas in Paramecium 

 and other unicellular animals. 1 What happened in the animals 

 he examined may be briefly summarised as follows: For 

 a large number of generations the unicellular animals multi- 

 plied by a process of simple division (" fissiferous bi-partition"). 

 When many generations, hundreds in some cases, had been 

 produced in this way, the individuals of the swarm were 

 seen to approach each other and come into contact, re- 

 maining attached to each other for some time. During 

 this period of conjugation the nuclei divided, and a 

 daughter nucleus from each of the individuals left the 

 organism in which it arose and migrated into the body of 

 the other partner in the conjugation. There the migrated 

 nucleus fused with the other daughter nucleus, which had 

 remained in the body of each animal. We see, therefore, 

 that one of these nuclei acted as a sperm cell, the other as 

 an egg cell. When this fusion had taken place, the two 

 unicellular organisms separated and a fresh period of simple 

 division took place, which again lasted for a great many 

 generations. Maupas also found that when he artificially 

 prevented these unicellular organisms from conjugating 

 they all degenerated, and that when conjugation did not 

 take place the race died out. It is true that the experi- 

 ments of subsequent observers have shown that the period 



1 Maupas, op. cit., p. 3 



