88 ELEMENTS OF BIOLOGY. 



of its vital functions no longer is repaired by a concurrem 

 and equivalent process of repair, it is clear that life cannot 

 be maintained for any length of time. If we are dealing 

 with a single definite part or organ, such as an individual 

 muscle, the result of this state of things is a progressive 

 *' atrophy." If the organ is not one necessary to life the 

 organism may survive, but the organ affected beconjes ulti- 

 mately unable to discharge its vital functions, and practically 

 dies, so far as the general economy is concerned. If a vital 

 organ is thus affected, or if the nutritive failure extends to 

 the entire organism, the final result, however long delayed, 

 is necessarily death. When an animal dies simply of " old 

 age," it is probably in consequence of this failure of nutri- 

 tion to supplement the incessant losses caused by living. 

 It is to be remembered, however, that we have undoubtedly 

 to deal here with a deeper law, not connected apparently 

 with the above. It might be thought that there is no reason 

 u;hy any animal should not live for an indefinite period, pro- 

 vided it could but maintain the standard of nutrition, so 

 that the losses of life should never for long exceed the 

 powers of repair. This, however, does not seem to be 

 even theoretically true. It seems, on the contrary, that 

 there is for every species of animal a certain comparatively 

 definite limit beyond which its life can not be prolonged. 

 It appears to be like a machine, " made " to run a certain 

 time, but certain to break down after reaching a given limit. 

 Some individuals of the species do not reach this limit ; 

 other individuals may exceed it ; but the limit remains for 

 the species as an " average period of life,'^ which a few over- 

 pass, whilst the majority never reach it. 



If, on the other hand, the process of repair exceeds that 

 of waste — if new material is added faster than old material 

 is destroyed — then we have the state of things which is pro- 

 perly termed growth. Growth may be of the organism as a 

 whole, or of any particular part or organ ; and in either case 

 it consists simply in the addition of matter similar in kind 



