AGE IN DAYS 
Figure 154 . — Females. (See explanation for Fig. 153 .) 
an upward sloping straight line. The relative 
strength of this force of mortality may be visualized 
in terms of the length of time required for a dou- 
bling of the mortality rate. 
Despite differences in heredity and environment 
this doubling of the mortality rate takes about 130 
to 1 35 days for male rats in each of the four popu- 
lations. Likewise the mortality rate doubles each 
130 to 135 days for the females in the population 
Davis studied as well as in Wiesner’s colony. This 
strongly suggests that physiological aging is a 
phenomenon independent of both heredity and 
environment. If physiological aging really is in- 
dependent of both heredity and environment we 
must perforce inquire as to the origin of the shorter 
interval of time required for the mortality rate to 
double for females in two of the populations. This 
interval was 90 days for the females in King’s 
colony during the entire life span, and only 47 
days for the females in Calhoun’s colony after 400 
days of age. 
Now, considering only the portions of the trends 
of mortality rates which consistently increase with 
age we find that mortality rate doubles as follows: 
Calhoun’s females, 47 days; King’s females, 90 
days; and all the remaining six sets of rats between 
130 and 135 days. 
Why females should be more variable in this 
characteristic than males can not be determined 
from these data. However, a phenomenon en- 
countered by Calhoun (97) suggests a probable 
reason. He studied four populations of Osborne- 
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