76 



H. S. SiMMS, B. N. Berg, and D. F. Davies 



The reason why the curves fall off after reaching a maximum 

 is simply that there are fewer remaining animals without 

 lesions of a given disease and which consequently can acquire 

 lesions for the first time. There is no decrease in the tendency 

 to form lesions among those individuals that have survived to 

 advanced age without them. This is shown in Fig. 4, where it 

 is seen that the tendency to form lesions increases progressively 

 with age. 



— . AGE IN DAYS 

 - 100 200 



— 1 r- 



^iL5 



3di 



^>- 



^20. 



Fig. 5. Logarithm of the probabihty of onset of new 

 lesions (among surviving rats having no lesions). Also 

 a curve for rat mortality (From Simms and Berg, 



1957). 



That this increase is a logarithmic function of age is shown 

 in Fig. 5, where the logarithm of the probability of onset 

 (among rats not having lesions) is plotted against age. These 

 curves approximate to straight lines. Note that they parallel 

 the bottom curve for mortality of rats. 



We may now compare rat and man. Fig. 6 contains two 

 charts each having data from both rat and man. At the left 

 end of the top chart are curves for occurrence of lesions in 

 rats. These are identical with the curves in Fig. 2, except 

 that here the time scale is very much condensed, making the 

 curves much steeper. Similarly, at the left end of the bottom 

 chart are curves for the distribution of onset of lesions in 



