212 BIOLOGY OF DEATH 



in which the environmental forces (of sub-lethal inten- 

 sity, of course) chiefly act in determining duration of 

 life, appears to be by changing the rate of metabolism of 

 the individual. Furthermore one would suggest, on this 

 view, that what heredity does in relation to duration of 

 life is chiefly to determine, within fairly narrow limits, 

 the total energy output which the individual can exhibit 

 in its life time. This limitation is directly brought about 

 presumably through two general factors: vis, (a) the 

 kind or quality of material of which this particular vital 

 machine is built, and (b) the manner in which the parts 

 are put together or assembled. Both of these factors 

 are, of course, expressions of the extent and character 

 of the processes of organic evolution which have given 

 rise to this particular species about which we may be 

 talking in a particular instance. 



There is some direct experimental evidence, small in 

 amount to be sure, but exact and pertinent, to the effect 

 that the duration of life of an animal stands in inverse re- 

 lation to the total amount of its metabolic activity, or put 

 in other words, to the work, in the sense of theoretical 

 mechanics, that it as a machine does during its life. 

 Slonaker kept 4 albino rats in cages like the old fashioned 

 revolving squirrel cages, with a properly calibrated odo- 

 meter attached to the axle, so that the total amount of 

 running which they did in their whole lives could be 

 recorded. The results were those shown in Table 26. 



It will be perceived that the amount of exercise taken 

 by these rats was astonishingly large. For a rat to 

 run 5,447 miles in the course of its life is indeed a re- 

 markable performance. Now these 4 rats attained an 

 average age at death of 29.5 months. But three control 

 rats confined in stationary cages so that they could only 



