Relation of Lifespan to Brain and Body Weight 129 



tend more and more to the view that the central nervous 

 system participates in the vegetative processes continually. 

 I therefore conclude that my assumption about the close 

 relation between brain development and overall precision of 

 physiological regulation is justified by our present knowledge. 

 Moreover, I am confident that this relation will find concrete 

 support when a quantitative comparative physiology comes 

 into being which will make possible direct tests of more specific 

 forms of this general assumption. 



Identification of the factors which determine the character- 

 istic ageing rates, and hence the lifespans of mammalian spec- 

 ies, is one of the central problems of gerontology. In recent 

 years several authors have proposed theories of ageing which 

 posit a relation of ageing to the spontaneous mutation rate 

 (Szilard, 1959) or to loss of information content (Yockey, 

 1958). Whatever other utility these theories may have, they 

 contribute little to our understanding of ageing, for their 

 authors fail to grasp the essential point that the spontaneous 

 mutation rate and the ageing rate are concomitant species 

 characters, so that to account for one in terms of the other is 

 merely to restate an observed fact. It would undoubtedly be 

 widely acknowledged that ageing must ultimately be accounted 

 for in terms of irreversible alterations in the molecular make- 

 up of organisms, and that gene mutations are an especially 

 important class of such irreversible molecular changes. Given 

 this consensus, the basic question is: why does the species 

 mutation rate tend to be proportional to the mean death rate, 

 or inversely proportional to the lifespan? I shall next discuss 

 this question in the light of the energy dissipation and fluctua- 

 tion hypotheses. 



The occurrence of irreversible molecular changes in bio- 

 logical systems can be exhaustively discussed under four 

 headings, as follows: 



(a) thermal denaturation — alterations of molecular struc- 

 ture that are essentially due to thermal energy, and occur at a 

 rate that is primarily dependent on the temperature; 



AGEING — ^V — 5 



