SURVIVORS DYING EACH 50 DAYS 
AGE IN DAYS 
Figure 153. — Males. 
Trends in death rate. The Davis data refer to wild gray rats living on a farm where both predation and disease were 
prevalent. The Calhoun data are that of the present study where predation was absent but disease and internal strife were 
present. The King data are that for wild gray rats bred for 25 generations in cages in groups of 4 to 7 rats. The Wiesner 
and Sheard data refer to domesticated Wistar albinos caged in pairs. There were approximately 100 rats of each sex in the 
Davis and Calhoun data, and approximately 1,000 rats of each sex in the King and in the Wiesner and Sheard data. The 
curves for King's data are extrapolated beyond 600 days of age on the basis of the trend exhibited prior to that time. 
for 1950 (89), there is a declining death rate be- 
tween 22 and 35 years of age. After 35 years of 
age the mortality rate increases rapidly. It cer- 
tainly looks as though increased experience with 
driving automobiles enhances our chances of avoid- 
ing involvement in a fatal accident. Beyond 35 
years of age some process of aging begins to counter- 
act the advantage gained through these prior years 
of experience. 
B. Increasing mortality rates with age. Beyond 
some age in each of the eight sets of data each 
increase in age has a higher mortality than the 
preceding one. In environments reduced to the 
barest minimum, that is the caged state of King’s 
rats and of Wiesner’s and Sheard’s rats, the aging 
phenomenon of increasing probability of death is 
evident from the time of weaning. 
Comfort (90) terms this phenomenon the “force 
of mortality”. When this force is predominating, 
the log of mortality rate plotted against age forms 
278 
