24 THE DURATION OF LIFE, [I. 



Let us imagine that one of the higher animals became im- 

 mortal ; 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 time, — a supposition which is of course hardly 

 conceivable. The individual would nevertheless be unable to 

 avoid, from time to time, slight injuries to one or another part 

 of its body. The injured parts could not regain their former 

 integrity, and thus the longer the individual lived, the more de- 

 fective and crippled it would become, and the less perfectly 

 would it fulfil the purpose of its species. Individuals are injured 

 by the operation of external forces, and for this reason alone it 

 is necessary that new and perfect individuals should continually^ 

 arise and take their place, and this necessity would remain even 

 if the individuals possessed the power of living eternally. 



From this follows, on the one hand, the necessity of repro- 

 duction, and, on the other, the utility of death. 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. 

 Hence by the operation of natural selection, the life of our 



the race would survive. But if individuals did not die they w^ould soon 

 multiply inordinately and would interfere 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 ofi" 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 v^^ould 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.] 



