4 B. Benjamin 



of the curve of deaths into component curves and that it was 

 questionable whether such subdivision could be theoretically 

 validated. Nevertheless he agreed that one important case of 

 sudden change of composition was the rapid transition between 

 the period of physical growth and the adult period. " In the 

 adult period we appear to be subject to a continually increas- 

 ing disorganisation or 'inability to withstand destruction' 

 while in childhood we appear on balance to gain organisation 

 or 'ability to withstand destruction'." 



While further work continued on the fitting of mathematical 

 functions to the force of mortality, [i^., attention was focused 

 on the curve of deaths (i.e. of {i^-h) ^s an alternative operand 

 by a paper by Phillips (1935), though this was not yet to be 

 taken up. Meanwhile, taking a closer look at the concept of 

 deterioration. Rich (1940) evolved a theory of mortality 

 based on an analogy between degrees of health and degrees of 

 temperature. This gave rise to a "health frequency distri- 

 bution" (a concept close to the present view of biological 

 normality as a dispersion of characteristics within broadly 

 separated limits), and changes of rating within this distribu- 

 tion could be regarded as forces of deterioration or of recupera- 

 tion. A "natural law of mortality" emerged from which 

 Makeham's and one of Perks's functions could be derived as 

 special cases. 



Clarke (1950) took up again the analysis of the curve of 

 deaths. He argued that mortality improvements had not 

 extended the natural lifespan but had only allowed more to 

 achieve it. He distinguished between "anticipated" and 

 "senescent" deaths; the ages at death in the latter group 

 were measures of natural lifespans and had a frequency dis- 

 tribution like other animal characteristics. His paper formed 

 the basis of the present work and his hypothesis will be re- 

 ferred to again presently. 



Beard (1950) also paid attention to the curve of deaths, 

 using the incomplete gamma function as the basic analytical 

 function. 



