282 THE SIGNIFICANCE OF SEXUAL REPRODUCTION 



certainty or precision from the facts with which we are at present 

 acquainted. The general solution of the problem will, however, 

 be found to lie in the conjugation of unicellular organisms, which 

 forms the precursor of true sexual reproduction. The coalescence 

 of two unicellular individuals which represents the simplest and 

 therefore probably the most primitive form of conjugation, must 

 have some directly beneficial effect upon the species in which it 

 occurs. 



Various assumptions may be made as to the nature of these bene- 

 ficial effects, and it will be useful to consider in detail some of 

 those suggestions which have been brought forward. Eminent 

 biologists, such as Victor Hene^n l and Edouard van Beneden 2 , 

 believe that conjugation, and indeed sexual reproduction generally, 

 must be considered as ' a rejuvenescence of life.' Biitschli also 

 accepts this view, at any rate as regards conjugation. These author- 

 ities imagine that the wonderful phenomena of life, of which 

 the underlying cause is still an unsolved problem, cannot be con- 

 tinued indefinitely by the action of forces arising from within 

 itself, that the clock-work would be stopped after a longer or 

 shorter time, that the reproduction of purely asexual organisms 

 would cease, just as the life of the individual finally comes to an 

 end, or as a spinning wheel comes to rest in consequence of friction, 

 and requires a renewed impetus if its motion is to continue. In 

 order that reproduction may continue without interruption, these 

 writers believe that a rejuvenescence of the living substance is 

 necessary, that the clock-work of reproduction must be wound up 

 afresh ; and they recognize such a rejuvenescence in sexual repro- 

 duction and in conjugation, or in other words in the fusion of two 

 cells, whether in the form of germ-cells or of two unicellular 

 organisms. 



Edouard van Beneden expresses this idea in the following words: 

 ' II semble que la faculte quo possedent les cellules, de se multiplier 

 par division soit limitee : il arrive un moment ou elles ne sont plus 

 capables de se diviser ulterieurement, a moins qu'elles ne subissent 

 le phenomene du rajeunissement par le fait de la fecondation. 



1 S. Hermann's 'Handbuch der Physiologie,' Theil II ; 'Physiologic der Zeugung,' 

 by V. H onsen. 



3 E. van Beneden, ' Recherches sur la maturation de I'oeuf, la fecondation et la 

 division cellulaire.' Gand u. Leipzig, 1 883, pp. 404 et seq. 



