STUDIES ON THE DURATION OF LIFE 213 



move about to a limited degree, but otherwise under 

 conditions, including temperature, identical with those 

 in the revolving cages, attained an average age at death 

 of 40.3 months. All were stated to have died of "old 



TABLE 26 

 Relation of longevity to muscular activity in rats (Slonaker) 



TOTAL NUMBER OF MILES BUN DUHINQ LIFE 



age. ' ' From this experiment it clearly appears that the 

 greater the tO|tal work done, or total energy output, the 

 shorter the duration of life, and vice versa. Or, put 

 another way, if the total activity per unit of time is in- 

 creased by some means other than increasing tempera- 

 ture, the same results appear as if the increased activity 

 is caused by increased temperature. It appears, in 

 short, to be activity per se, and not the temperature per 

 se that is of real significance. There is other evidence, 

 for which space lacks here, pointing in the same direction. 

 An entirely different, and extremely suggestive line 

 of evidence in favor of the view hei'e set forth, has been 

 given by Professor Max Rubner, the distinguished Ger- 

 man student of the energy relations of the living organ- 

 ism. Studying a considerable range of animals, he has 

 found that all transform nearly the same total amount 

 of energy, per Jcilo of body weight, in the whole period 

 from their birth to their natural death. The mean value 

 of the constant Rubner finds to be 191,600 calories, the 

 values for different species ranging between 141,090 and 



