v.] IN THE THEORY OF NATURAL SELECTION. 289 



constitute the causes which led to its first appearance. Sexual 

 reproduction came into existence before it could lead to here- 

 ditary individual variability. Its first appearance must therefore 

 have had some other cause ; but the nature of this cause can 

 hardly be determined with any degree of 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 

 beneficial effects, and it will be useful to consider in detail 

 some of those suggestions which have been brought forward. 

 Eminent biologists, such as Victor Hensen^ and Edouard van 

 Beneden^, believe that conjugation, and indeed sexual repro- 

 duction generally, must be considered as ' a rejuvenescence of 

 life.' Biitschli also accepts this view, at any rate as regards 

 conjugation. These authorities imagine that the wonderful 

 phenomena of life, of which the underlying cause is still an 

 unsolved problem, cannot be continued 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 repro- 

 duction 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 ne- 

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

 afresh ; and they recognize such a rejuvenescence in sexual 

 reproduction and in conjugation, or in other words in the fusion 

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

 cellular organisms. 



^ S. Hermann's ' Handbuch der Physiologic,' Theil II ; ' Physiologic 

 der Zeugung,' by V. Hcnsen. 



2 E. van Beneden, ' Rechcrches sur la maturation do roeuf, la fecon- 

 dation et la division cellulaire.' Gand u. Leipzig, 1883, pp. 404 ct seq. 



U 



