1 66 SENESCENCE AND REJUVENESCENCE 



growing animals of the same size as regards rate of metabolism, 

 they differ widely from these in their capacity for acclimation. This 

 difference raises the question whether capacity for acclimation is 

 a fundamental or only an incidental feature of the age cycle. If 

 it is a fundamental feature, then the reduced animals have under- 

 gone rejuvenescence only in certain respects and have actually 

 become older physiologically in certain other respects. If, on the 

 other hand, it is merely incidental, then the reduced animals have 

 undergone what is essentially rejuvenescence and merely require 

 food in order to make them identical with young, growing individuals. 

 The latter alternative seems to be the correct one. If the decrease 

 in capacity for acclimation during starvation is regarded as a 

 process of senescence, it becomes necessary to admit that an animal 

 which is old in this respect may become young within a few hours 

 when it is fed. The susceptibility as measured by the direct method 

 and the rate of carbon-dioxide production are certainly much more 

 adequate criteria of physiological age and condition than the 

 capacity for acclimation. In other words, reduction by starva- 

 tion is essentially a process of rejuvenescence in these animals, 

 and the difference between them and young, growing animals as 

 regards capacity for acclimation is an incidental rather than a 

 fundamental difference. 



When the animal reduced by starvation is again fed, its physio- 

 logical condition very soon becomes indistinguishable from that of 

 growing animals of about the same size. In the advanced stages 

 of reduction the susceptibility of the reduced animal is almost always 

 somewhat greater than that of fed animals of the same size, and the 

 effect of renewed feeding is a decrease in susceptibility to about 

 the same level as that of the fed animal. The capacity for accli- 

 mation, as already noted, increases even after a single feeding, 

 but in advanced stages of reduction by starvation several feedings 

 are usually necessary, i.e., the animal must attain a well-fed con- 

 dition before the capacity for acclimation is equal to that of grow- 

 ing animals. The effect of a single feeding may appear within an 

 hour or two, but lasts at most only a few days, the animal rapidly 

 returning to the completely starved condition. But if other feed- 

 ings follow at sufficiently short intervals, growth soon begins, and 



