Biological Approach in Study of Ageing 7 



since in each case the large variance is an expression of poor 

 homeostasis, and this in turn may represent a partial with- 

 drawal of natural selection. 



The variance of single characters has been used as a 

 measure of inbreeding depression (Maclaren and Michie, 1954, 

 1955) and it might in some instances be convenient to use 



5000 



10 20 30 40 50 50 70 

 AGE AT DEATH (Yr.) 



Fig. 1. Distribution by age of deaths from all causes (white 



U.S. males, 1939-41) and of pedestrian deaths in road accidents 



(England and Wales, 1950). (Reproduced, with permission, 



from Comfort, 1956. Lancet, ii, 773.) 



some such measure of the variance of a number of characters 

 in estimating senescence. As in inbred strains, however, the 

 characters whose variance increases with age are quite irreg- 

 ularly distributed between species and between genetic races of 

 the same species. By measuring the probability of death from 

 the sum total of causes we are using a measure applicable to all 

 organisms, which can be treated mathematically by con- 

 venient methods, and which is experimentally consistent — 

 often to a very high degree. In man and in some inbred lines 



