8 A. Comfort 



the pattern of mortality can be substantially alfected by 

 diseases which have a characteristic age distribution. But we 

 can see that the crude survival and mortality curves are 

 valid measures of vigour against forces other than these if we 

 compare the distribution of human pedestrian deaths in road 

 accidents (Fig. 1). This depends upon the decline of several 

 characters of vigour; sensory acuity, reaction time, motor 

 co-ordination and recovery after injury. Apart from the fact 

 that we normally exclude infant mortality from the treatment 

 of senescence, we need not therefore follow Pearson (1897) in 

 adopting the five separate Deaths which formed the frontis- 

 piece to the first of these Colloquia on Ageing, except in strains 

 of animals or circumstances of culture where one of them 

 grossly predominates. 



Animal life tables 



If we had a comprehensive account of the relation between 

 growth, development, mortality and chronological age in a 

 sufficient range of representative species, the truth of most of 

 the general theories which have been put forward to explain 

 senescence could probably be tested by inspection. The 

 actuarial studies which we already have, combined with 

 maximum age records, suggest that in invertebrates there is 

 an inverse relationship between degree of cell replacement and 

 liability to senile change. It is now of great importance to 

 obtain accurate data for the relation of age to mortality in the 

 main types of vertebrates, and in as many different species as 

 possible. McCay and co-workers showed (1943) that ageing 

 and general metabolic activity in rats are dissociable, at least 

 prior to maturity, as growth and development are dissociable 

 in the tadpole; it is therefore quite one of the most important 

 problems of age studies to find out, if possible, which compon- 

 ents of the developmental "programme" in mammals deter- 

 mine the timing of senescence. Given a sufficient range of 

 comparative information, we might expect to answer this 

 question either directly, from observed correlations, or by a 

 relatively small number of fundamental experiments. 



