Discussion 135 



and most of the large ones are ungulates. It would be interesting 

 to know whether, if you calculate your coefficients just on rodents, 

 just on primates, and just on ungulates, you would get results which 

 are at least approximately consistent with those you get on the 

 whole sample combined. 



Sacher: I only know this qualitatively and graphically. The 

 values of the allometric coefficients vary considerably from order to 

 order. The goodness of fit would have been greatly improved if I 

 had omitted the ungulates, because they have quite different 

 allometric relations of body weight to brain weight, and in effect 

 they made the overall relation poorer. 



Rotblat: I was very glad to see that you describe life processes in 

 terms of numbers. I was a little bit disturbed, however, when you 

 ended up by introducing a term which cannot be expressed in 

 numbers, namely stability or adaptability of physiological function. 

 Can you put this in some kind of quantitative relation to the index 

 of cephalization, or the mitotic activity ? Have you any indication 

 that there may be some ways of increasing lifespan by increasing 

 stability ? 



Sacher: The term " stability " in my thinking is a general term that <^ 

 subsumes all the properties that have to do with mortality, stress 

 resistance, length of life, etc. In this sense, an animal that lives 

 longer has greater stability, and a species that has a lower mortality 

 rate for a given disease or stress has greater stability. In other 

 words, stability is a general physical character of organisms. If one 

 holds the point of view that all these things are fated, determined in 

 advance by the genotype, then there is no reason for talking about 

 stability. But if one thinks of organisms as dynamic functioning 

 systems whose probabilities of failure arise from their function 

 (Sacher, 1956, 1958), then stability is a natural term and it becomes 

 reasonable to think of improving the stability characteristic of 

 organisms. We cannot replace any of our bodily elements with 

 better ones, as an engineer can replace vacuum tubes, but it might 

 be possible (remembering that the central nervous system is im- 

 plicated in every physiological activity, and that these responses 

 are conditionable) eventually to evolve a kind of ontogeny and con- 

 ditioning that would make for more stable physiological functioning 

 in the given environment. 



Rotblat: This seems to be going around in circles. You have ex- 

 plained the span of life in terms of stability and vice versa. It seems 

 to me that mutation rate is a quantity outside the circle, because you 

 can say that there is something which goes on all the time inde- 

 pendently of us. 



