I.] THE DURATION OF LIFE. 23 



sible— can only happen by the production of new shoots, i. e. 

 by an increase in the number of cell-generations. The process 

 is not so obvious in animals, because in them the formation of 

 3'oung cells does not lead to the production of new and visible 

 parts, for the new material is merely deposited in the place of 

 that which is worn out and disappears. Among plants, on the 

 other hand, the old material persists, its cells become lignified, 

 and it is built over by new cells which assume the functions of 

 life. 



It is certainly true that the question as to the necessity of 

 death in general does not seem much clearer from this point of 

 view than from the purely physiological one. This is because we 

 do not know why a cell must divide 10,000 or 100,000 times and 

 then suddenl}^ stop. It must be admitted that we can see no 

 reason why the power of cell-multiplication should not be 

 unlimited, and why the organism should not therefore be 

 endowed with everlasting life. In the same manner, from 

 a physiological point of view, we might admit that we can see 

 no reason why the functions of the organism should ever cease. 



It is only from the point of view of utility that we can under- 

 stand the necessity of death. The same arguments which were 

 employed to explain the necessity for as short a life as possible, 

 will with but slight modification serve to explain the common 

 necessity of death \ 



[^ After reading these proofs Dr. A. R. Wallace kindly sent me an un- 

 published note upon the production of death by means of natural selection, 

 written by him some time between 1865 and 1870. The note contains 

 some ideas on the subject, which were jotted down for further elabora- 

 tion, and were then forgotten until recalled by the argument of this 

 Essay. The note is of great interest in relation to Dr. Weismann's 

 suggestions, and with Dr. Wallace's permission I print it in lull below. 



' The Action of Natural Selection in producing Old Age, 



Decay, and Death. 



' Supposing organisms ever existed that had not the power of natural 

 reproduction, then since the absorptive surface would only increase as 

 the square of the dimensions while the bulk to be nourished and renewed 

 would increase as the cube, there must soon arrive a limit of growth. 

 Now if such an organism did not produce its like, accidental destruction 

 would put an end to the species. Any organism therefore that, by acci- 

 dental or spontaneous fission, could become two organisms, and thus 

 multiply itself indefinitely without increasing in size be3^ond the limits 

 most favourable for nourishment and existence, could not be thus exter- 

 minated : since the individual only could be accidentally destroyed, — 



