THE DUKATION OF LIFE. 23 



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 l . 



Let us imagine that one of the higher animals became immortal ; 

 it then becomes perfectly obvious that it would cease to be of 

 value to the species to which it belonged. Suppose that such an 

 immortal individual could escape all fatal accidents, through infinite 



[* After reading these proofs Dr. A. E. Wallace kindly sent me an unpublished 

 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 elaboration, 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 full 

 below. 



' THE ACTION OF NATURAL SELECTION IN PRODUCING OLD AGE, 



DECAY, AND DEATH. 



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

 tion, 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 accidental or spontaneous fission, could become two organisms, 

 and thus multiply itself indefinitely without increasing in size beyond the limits 

 most favourable for nourishment and existence, could not be thus exterminated : 

 since the individual only could be accidentally destroyed, the race would survive. 

 But if individuals did not die they would soon multiply inordinately and would inter- 

 fere with each other's healthy existence. Food would become scarce, and hence the 

 larger individuals would probably decompose or diminish in size. The deficiency of 

 nourishment would lead to parts of the organism not being renewed ; they would 

 become fixed, and liable to more or less slow decomposition as dead parts within a 

 living body. The smaller organisms would have a better chance of finding food, the 

 larger ones less chance. That one which gave off several small portions to form 

 each a new organism would have a better chance of leaving descendants like 

 itself than one which divided equally or gave off a large part of itself. Hence it 

 would happen that those which gave off very small portions would probably soon 

 after cease to maintain their own existence while they would leave a numerous 

 offspring. This state of things would be in any case for the advantage of the race, 

 and would therefore, by natural selection, soon become established as the regular 

 course of things, and thus we have the origin of old age, decay, and death ; for it is 

 evident that when one or more individuals have provided a sufficient number of 

 successors they themselves, as consumers of nourishment in a constantly increasing 

 degree, are an injury to those successors. Natural selection therefore weeds them 

 out, and in many cases favours such races as die almost immediately after they have 

 left successors. Many moths and other insects are in this condition, living only to 

 propagate their kind and then immediately dying, some not even taking any food 

 in the perfect and reproductive state.' E. B. P.] 



