198 THE PHYSIOLOGY OF REPRODUCTION 



representing the two chief theories that have been elaborated (with 

 some modifications by their respective adherents) to explain the 

 observed phenomena. 1 According to one hypothesis, conjugation 

 of the gametes results in a rejuvenescence which is essential for the 

 perpetuation of the race (see p. 221). According to the second 

 theory, which is not necessarily antagonistic to the first, gametic 

 union is a source of variation.' 2 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 3 adopted the same idea, and Weismann made 

 it the basis of his famous theory of heredity. 4 " 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 reproduc- 

 tive 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 pro- 

 duction 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 together to 

 form more complex units, named determinants, which represent the 

 separate parts of the organism. The determinants are supposed to 



1 For accounts of the various theories which have been put forward con- 

 cerning the nature of fertilisation, see Wilson, loc. cit., Geddes and Thomson, 

 The Evolution of Sex, 2nd Edition, London, 1901 ; Weismann, The Evolution 

 Theory, English Translation, London, 1904 ; and Lock, Variation, etc., London, 

 1908. Further references are given in these works. 



2 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 constantly 

 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 

 Coniugazione e il differenziamento sessuale negli Infusori," Arch. f. Protiati'it- 

 kunde, 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. 



3 Brooks, The Law of Heredity, Baltimore, 1 883. 



4 Weismann, The Germ- Plasm, English Translation, London, 1893. 



