v.] IN THE THEORY OF NATURAL SELECTION, 29 1 



one. If indeed we could assume that each animal contained 

 half the power necessary for reproduction, then both together 

 would certainly form an efficient whole ; but it is hardly pos- 

 sible to apply the term rejuvenescence to a process which is 

 simply an addition, such as would be attained under other 

 circumstances by mere growth ; neglecting, for the present, 

 that factor which, in my opinion, is of the utmost importance in 

 conjugation, — the fusion of two hereditary tendencies. If re- 

 juvenescence possesses any significance at all, it must be this, 

 — that by its means a force, which did not previously exist 

 in the conjugating individuals, is called into activity. Such 

 a force would, however, owe its existence to latent energy 

 stored up in each single animal during the period of asexual 

 reproduction, and such latent forces would necessarily be of 

 different natures, and of such a constitution that their union at 

 the moment of conjugation would give rise to the active force of 

 reproduction. 



The process might perhaps be compared to the flight of two 

 rockets, which by the combustion of some explosive substance 

 (such as nitro-glycerine) stored up within themselves are im- 

 pelled in such a direction that they would meet at the end of 

 their course, when all the nitro-glycerine had been completely 

 exhausted. The movement would then come to an end, unless 

 the explosive material could have been meanwhile renewed. 

 Now suppose that such a renewal were achieved by the forma- 

 tion of nitric acid in one of the rockets and glycerine in the 

 other, so that when they came into contact nitro-glycerine 

 would be formed afresh equal in quantity and in distribution on 

 both the rockets to that which was originally present. In this 

 way the movement would be renewed again and again with 

 the same velocity, and might continue for ever. 



Rejuvenescence can be rendered intelligible in theory by 

 some such metaphor, but considerable difficulties are encoun- 

 tered in the rigid application of the metaphor to the facts of the 

 case. In the first place, how is it possible that the motive force 

 can be exhausted by continual division, while one of its com- 

 ponents is being formed afresh in the same body and during 

 the same time ? When thoroughly examined the loss of the 

 power of division is seen to follow from the loss of the powers 

 of assimilation, nutrition, and growth. How is it possible that 



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