XII.] CONJUGATION AND SEXUAL REPRODUCTION. 107 



convolutions of the brain, and giving rise to the foetus under the 

 influence of the semen ; just as the brain, under the influence of 

 external impressions, gives rise to thoughts. The term ' con- 

 ception,' when figuratively applied to mental processes,— a term 

 which has been obviously derived from conception on the part 

 of a woman, — is here reversed, and used to explain the very 

 process from which it is itself derived. 



The same fundamental idea runs through all theories of 

 fertilization up to the present time — the idea that the fertili- 

 zation, i.e. the ' vitalization of the ^g^^ is the important part, 

 or, as we may say, the true purpose of sexual reproduction. 

 The other side of this mode of reproduction has been com- 

 paratively neglected ; the fact that two different predispositions, 

 on the one hand that of the father, and on the other that of 

 the mother, are by fertilization united in a single organism, 

 has appeared as a secondary, but it is clear to some extent 

 as an inevitable result of fertilization. Although this view is 

 nowhere directly expressed, it is implied in all the utterances 

 of both older and more recent writers. It must be admitted 

 that so long as biologists were acquainted with no method of 

 reproduction except the sexual, it was impossible to regard 

 fertilization in any other light ; it seemed that the co-operation 

 of two individuals was indispensable in order to call a third 

 to life, and it can scarcely have been surprising for this new 

 organism to resemble its progenitors more closely than any 

 other living being. But, even in recent times, when other 

 methods of reproduction among plants and animals gained 

 recognition, they did not at first cause any alteration in that 

 view which regards fertilization as a process of vitalization, a 

 calling forth of new hfe. In the case of all those higher beings 

 which do not possess the power of asexual reproduction, it 

 became evident that a certain complexity of organization ex- 

 cluded this method of increase. But then the asexual repro- 

 duction of the lower organisms is by no means always sufficient 

 to fulfil every condition necessary for the maintenance of the 

 species, and hence the origin of new individuals from unicellular 

 germs capable of fertilization must have appeared as an essen- 

 tial advantage. 



The first fact which tended to throw doubt on the view that 

 fertilization is a renewal of life was the discovery of parthe- 



