136 LIFE AND DEATH, [III. 



from any absolute intrinsic necessity inherent in the nature of 

 living matter, but on grounds of utility, that is from necessities 

 which sprang up, not from the general conditions of life, but 

 from those special conditions which dominate the life of multi- 

 cellular organisms. If this were not so, unicellular beings 

 must also have been endowed with natural death. I have 

 already expressed these ideas elsewhere \ and have briefly 

 indicated how far, in my opinion, natural death is expedient 

 for multicellular organisms. I found the essential reason for 

 confining the life of the Metazoa to a fixed and limited period, 

 in the wear and tear to which an individual is exposed in the 

 course of a life-time. For this reason, ' the longer the individual 

 lived, the more defective and crippled it would become, and 

 the less perfectly would it fulfil the purpose of its species ' 

 (1. c, p. 24). Death seemed to me to be expedient since 'worn- 

 out individuals are not only valueless to the species, but they 

 are even harmful, for they take the place of those which are 

 sound' (1. c, p. 24). 



I still adhere entirely to this explanation ; not of course in 

 the sense that an actual physical struggle has ever taken place 

 between the mortal and immortal varieties of any species. If 

 Gotte understood me thus, he may be justified by the brief 

 explanations given in the essay to which I have alluded ; but 

 when he also attributes to me the opinion that such hypo- 

 thetically immortal Metazoa had but a very limited period for 

 reproduction, I fail to see what part of the essay in question 

 can be brought forward in support of his statement. Only 

 under some such supposition can I be reproached with having 

 assumed the existence of a process of natural selection which 

 could never be eftective, because any advantage which accrued 

 to the species from the shortening of the duration of life could 

 not make itself felt in a more rapid propagation of the short-lived 

 individuals. The statement 'that in this and in every other 

 case it is a sufficient explanation of the processes of natural 

 selection to render it probable that any kind of advantage is 

 gained '^ is indeed erroneous. The explanation ought rather to 

 be ' that the forms in question would for ever transmit their 

 characters to a greater number of descendants than the other 



^ See the first essay upon * The Duration of Life,' p. 23 et seq. 

 ^ ' Ursprung des Todes,' p. 29. 



