294 SIGNIFICANCE OF SEXUAL REPRODUCTION [V. 



together with all the hereditary tendencies contained in them ; 

 but although it is certain that nutrition in the proper sense 

 of the word cannot take place, because neither of the animals 

 receives an addition of liquid food by the coalescence, yet 

 the consequence of this process must be in one respect similar 

 to that of nutrition and growth : — the mass of the body and the 

 quantity of the forces contained in it undergo simultaneous 

 increase. It is not inconceivable that effects are by this means 

 rendered possible, which under the peculiar circumstances 

 leading to conjugation, could not have been otherwise produced. 



I believe that this is at any rate the direction in which we 

 shall have to seek for the first meaning of conjugation and 

 for its phyletic origin. This first result and meaning of con- 

 jugation may be provisionally expressed in the following 

 formula : — conjugation originally signified a strengthening of 

 the organism in relation to reproduction, which happened when 

 from some external cause, such as want of oxygen, warmth, or 

 food, the growth of the individual to the extent necessary for 

 reproduction could not take place. 



This explanation must not be regarded as equivalent to that 

 afforded by the theory of rejuvenescence ; for the latter process 

 is said to be necessary for the continuance of reproduction, and 

 ought therefore to occur periodically quite independently of 

 external circumstances ; while according to my theory, con- 

 jugation at first only occurred under unfavourable conditions, 

 and assisted the species to overcome such difficulties. 



But whatever the original meaning of conjugation may have 

 been, it seems to have become already subordinated in the 

 higher Protozoa, as is indicated by the changes in the course 

 taken by this process. The higher Protozoa when conjugating 

 do not as a rule coalesce completely and permanently^ in the 

 manner followed by the lower Protozoa, and it seems to me 

 possible, or even probable, that in the former the process has 

 already gained the full significance of sexual reproduction, and 

 is to be looked upon as a source of variability. 



Whether this be so or not, I believe it is certain that 

 sexual reproduction could not have been entirely abandoned at 

 any period since the time when the Metazoa and Metaphyta 



' Coalescence takes place in the so-called bud-like conjugation of 

 Vorticellidae and Trichodinidae, etc. 



