LIFE AND DEATH. 135 



natural death is expedient for multicellular organisms. I found 

 the essential reason for confining 1 the life of the Metazoa to a 

 fixed and limited period, in the wear and tear to which an indi- 

 vidual 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 under- 

 stood 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 hypothetically 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 effective, 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 ' l 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 forms.' I have not 

 however as yet attempted to think out in detail such processes of 

 natural selection as would limit the somatic part of the Metazoan 

 body to a short term of existence, and I only wished to emphasize 

 the general principle lying at the basis of the whole process, with- 

 out stating the precise manner in which it operates. 



If I now attempt to take this course, and to reconstruct theo- 

 retically the gradual appearance of natural death in the Metazoa, 

 I must begin by again alluding to Gotte's criticisms in reference 

 to the operation of natural selection. 



1 ' Ursprvmg des Todes,' p. 29. 



