Growth and Senescence 



by Medawar (1942). Both in whole animals and in specific 

 organs and tissues the rate of growth declines throughout life. 

 Medawar points out that it is not self-evident that this decline 

 is the result of active growth inhibition. 'We are so deeply 

 influenced by the spirit of Newton's First Law that we tend to 

 think that whenever a rate falls off, something is actively sup- 

 pressing it. This is true of rates of motion, but it is not true in 

 quite the same sense of the rates of a type of change which we 

 may call changes in probability states. The rate at which heat is 

 lost from a cooling body is initially high, and falls off as its 

 temperature approaches that of the environment. The rate at 

 which the distribution of molecules in a closed diffusion system 

 tends towards uniformity is likewise rapid at first, and slower 

 and slower thereafter. In these cases, and in others similar to 

 them, we are dealing with rates that fall off "of their own 

 accord", with systems that tend to a certain, most-probable state 

 at a rate which depends upon how far they have yet to go to 

 reach it. We may look in vain for inhibitors and controllers: 

 they are not there. I do not know whether what I have called 

 the "kinetic picture" of growth will be found to fall within the 

 domain of statistical mechanics. ... It is simply a picture which 

 we should keep in mind when thinking of growth processes, lest 

 we should come to regard the doctrine of growth-controlling 

 factors as self-evident; which it certainly is not.' 



This argument is graphical rather than explanatory, and the 

 analogy which it contains must be approached with caution. It 

 is evident that organ size has certain properties of an equili- 

 brium state, in approaching which the cell number and growth 

 energy vary after the manner of potential energy in the process 

 of redistribution. The equilibrating forces, however, mani- 

 festly arise, on the evidence of explanation, from the organ's and 

 the cell's surroundings. Mathematically similar systems involv- 

 ing real energy loss, such as cooling, are in no real sense analog- 

 ous, since they are examples of a process not subject to further 

 analysis. The decline in human population-growth is as fair a 

 comparison. Although morphogenesis is no doubt ultimately 

 expressible as a redistribution of energy, 'inherent' decline of 

 rate in approaching a most-probable state is only explanatory, in 

 the sense of providing a satisfactory regression of causes to the 



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