REPRODUCTION AND SEX 193 



exist separately in the body, (c) that there is nothing in 

 these two processes which makes it necessary for them 

 always to be thus connected. On the one hand, for 

 instance, we get in parthenogenesis an isolation of germ- 

 substance without conjugation, and on the other hand 

 in Polytoma, where the body of ordinary individuals 

 answers the requirements of a gamete, and perhaps also 

 in the Sporozoa, conjugation is unaccompanied by any 

 separation of germ-substance from body-substance. Of 

 these two features of sexual reproduction, one, the isolation 

 of the germ-substance, need not be considered further. 

 Its occurrence from time to time appears to be a necessity 

 of life in animals which have distinct body nuclei, and in 

 these animals it is effected either in the course of sexual 

 reproduction or by parthenogenesis, but it is not the primary 

 object of sexual reproduction. To regard it as such would 

 be to leave unexplained the occurrence of conjugation 

 during that process. We have already shown why the isola- 

 tion of germs necessarily forms part of most acts of sexual 

 reproduction, — if, indeed, that process does not always, 

 even in the Protozoa, involve something of the kind, — 

 namely, that in these acts it is only after the freeing of the 

 germs that conjugation, the essential feature of the sexual 

 process, can take place. 



There remains the problem of the meaning of conjuga- 

 tion. Unfortunately it is not an easy one to 

 The Problem solve. Conjugation always consists in the fusion 

 tion. njuea " of two energids known as gametes, whose nuclei 

 and cytoplasm so unite that there results a 

 single energid called a zygote. Gametes are generally 

 germs, but in some Protozoa, as in Polytoma, they are 

 adult bodies. In itself, conjugation is an act entirely dis- 

 tinct from reproduction, and has a precisely opposite effect, 

 since it lessens the number of individuals by fusion, 

 whereas reproduction increases them by division. When 

 it takes place between adult bodies it has no connection 

 with reproduction. But when it takes place between 

 germs it becomes, owing to the inability of such germs to 

 complete their development without it, a part of the repro- 

 ductive process, even though it halves the number of 

 adults that the germs could produce if they developed 



