192 THE PHYSIOLOGY OF REPRODUCTION 



one hypothesis, conjugation of the gametes results in a re- 

 juvenescence which is essential for the perpetuation of the race 

 (see p. 212). According to the second theory, which is not 

 necessarily antagonistic to the first, gametic union is a source 

 of variation. 1 The latter theory may now be briefly considered. 

 A full discussion of the hereditary effects of fertilisation is, 

 however, beyond the scope of the present work. 



The doctrine that conjugation is a source of variation was 

 first promulgated at the beginning of the last century by 

 Treviranus. Subsequently Brooks 2 adopted the same idea, and 

 Weismann made it the basis of his famous theory of heredity. 3 

 " Sexual reproduction is well known to consist in the fusion of 

 two contrasted reproductive cells, or perhaps even in the fusion 

 of their nuclei alone. These reproductive cells contain the 

 germinal material or germ-plasm, and this again, in its specific 

 molecular structure, is the bearer of the hereditary tendencies 

 of the organisms from which the reproductive cells originate. 

 Thus, in sexual reproduction, two hereditary tendencies are in 

 a sense intermingled. In this mingling I see the cause of the 

 hereditary individual characteristics ; and in the production of 

 these characters, the task of sexual reproduction. It has to 

 supply the material for the individual differences from which 

 selection produces new species." 



Weismann supposes the nuclear chromatin of the cell to 

 consist of a large number of self-propagating vital units which 

 he calls biophors. These biophors he believes to be grouped 



1 A third theory, which has never obtained any great support among 

 biologists, suggests that the purpose of sexual reproduction may be to prevent 

 variation, and so preserve specific uniformity. According to this view the 

 sexual process, although continually creating new variations, is also con- 

 stantly obliterating them by tending to produce individuals possessing the 

 mean of their parents' characters. This theory, which is the converse of the 

 second theory referred to in the text, has received the support of the Hertwigs. 

 In this connection it may be remarked that variability is quite as great among 

 non-sexual parthenogenetic animals as among those which are reproduced 

 sexually. This fact is difficult to explain if we adopt the theory that the purpose 

 of gametic union is to induce variability. Moreover, Enriques (" La Con- 

 iugazione e il differenziamento sessuale negli Infusori," Arch. f. Protisten- 

 knnde, vol. ix., 1907), as a result of a series of experiments upon conjugation 

 in Infusoria, has adopted a similar view to that of the Hertwigs. 



2 Brooks, The Law of Heredity, Baltimore, 1883. 



* Weismann, The Germ Plasm, English Translation, London, 1893. 



