Relation of Lifespan to Brain and Body Weight 131 



that the rate of accretion of ageing injury in mammahan 

 species increases as the metabohc rate increases and decreases 

 as precision of physiological regulation increases. Therefore 

 the irreversible changes that underlie ageing are not necessarily 

 and inevitably determined by the rate of dissipation of 

 energy. The intervention of improved physiological regula- 

 tions, by maintaining the average value of the milieu interieur 

 nearer to the biochemically ideal, and by reducing the 

 magnitude of the fluctuations away from this average, acts 

 to reduce the probability per unit time of irreversible changes, 

 and thus to reduce the rate of ageing. This has been a highly 

 significant feature in the evolution of mammals, and it 

 remains to be seen whether man can take conscious advantage 

 of this principle to bring about a real decrease in his rate of 

 ageing. 



Appendix 1. Relation of log lifespan to log body weight, log brain 

 weight, index of cephalization and log specific metabolic rate. Also 

 mean values, variances, and standard errors of the coefficients*. 



a. Mean values and standard errors of the variables, and their 

 variances. 



Log lifespan (years) ^ = 1 -283 ± -042 SI = -10900 



Log body weight (grams) ^ = 4-099 ± -163 SI = 1 -67158 



Log brain weight (grams) 2 =1-841 ±-113 SI = -80666 



Index of cephalization w= 0+ -032 S^ = -06570 



b. Log lifespan (x) on log body weight (y). See Equation (2). 



b^ = -198 ± -021 Slj, = -04402 



* The total regression of variable x on variable y is denoted by bxj,. The 

 corresponding regression equation is 



X — X = bxy(y — y) 



The partial regression of x on y, independent of the regressions of x and y on a, 

 third variable, z, is denoted by bxp.z. The multiple regression equation is of the 

 form 



X - X = hxy.z{y -y) + h^y.ziz - z) 



The total variance of variable x is denoted by S^. The variance of x that is not 

 accounted for by regression on the other variable or variables in the regression 

 equation is denoted by S^y, S^^y etc. 



