196 AMPHIMIXIS OR ESSENTIAL MEANING OF [XII. 



We know that, up to the present time, fertihzation has been 

 regarded as a vitalizing process, without which the development 

 of the ^%g either cannot occur at all, or only exceptionally. I 

 need not repeat what I have already said upon this idea in the 

 first part of the essay, and it is not necessary to follow the 

 gradual modifications which have been introduced ; but I should 

 like to submit to a trial the support which the upholders of these 

 views have always sought in the process of conjugation, and 

 which they are still seeking to-day. 



Maupas, the able investigator of the vital processes of Infu- 

 soria, considers that the effect of conjugation is such as to ensure 

 the continuation of the species ; it imparts to the animal the 

 power ' de renouveler et rajeunir les sources de la vie.' Hence, 

 according to this view, the most profound significance of con- 

 jugation is to be found in rejuvenescence, an idea which was 

 long ago accepted and applied, sometimes to fertilization, some- 

 times to conjugation, and sometimes to both together, by Biitschli, 

 Engelmann, Hensen, E. van Beneden, and more recently by R. 

 Hertwig. Maupas also looks upon these two processes as es- 

 sentially similar, and regards both as a 'rejuvenescence,' 

 without which life would, sooner or later, come to an end. He 

 sharply distinguishes between this somewhat mystical rejuven- 

 escence and that which consists in the renewal of many of the 

 external parts of the animal, such as mouth, bristles, cilia, &c. 

 Such regeneration is certainly connected with conjugation, but 

 it also occurs at every fission of an Infusorian and cannot there- 

 fore be an essential part of the former process. The rejuven- 

 escence which Maupas regards as the essence of conjugation is 

 something entirely different, and can hardly be described except 

 as a ' renewal of vital force,' using the expression in the sense 

 of the old natural philosophers. All other attempted definitions 

 of this rejuvenescence are vague and unsatisfactory. It may 

 well be doubted whether the return to a certain form of ' vital 

 force' is in harmony with the physiology of to-day. On the 

 other hand, no period of time has been entirely without an 

 advocate of this principle, and quite recently the accomplished 

 physiologist Bunge has, although with much reserve, again 

 supported the ancient belief in a vital force. In any case we 

 could only accept this idea if it were shown that there is no 

 chance of explaining the phenomena of life, even in principle. 



