THE PROBLEM AND METHODS OF INVESTIGATION 85 



reduction from starvation the rate of metabolism gradually rises, 

 so that a starved animal, reduced to, let us say, one-half its size at 

 the beginning of the experiment, has a much higher rate of metab- 

 olism than well-fed animals of its original size and about the same 

 rate as well-fed animals of its reduced size. But the reduced animal 

 has to a large extent lost its ability to become acclimated to depress- 

 ing agents and conditions, and in spite of its high rate of metabolism 

 is more susceptible to low concentrations of cyanide, alcohol, etc., 

 and also to low temperatures, than well-fed animals of the same size 

 as itself, and shows about the same susceptibility as well-fed animals 

 of its original size, although these possess a much lower rate of 

 metabolism. In other words, the animal which is using its own 

 structural substance as a source of energy is much less able to 

 acclimate itself to depressing conditions than an animal with the 

 same rate of metabolic reaction but with abundant nutritive ma- 

 terial. Consequently, it is impossible to determine the differences 

 in rate of metabolism between well-fed and starved animals by the 

 indirect method. 1 



In some cases also, where the differences of size between animals 

 compared are very great, the smaller animals die of starvation 

 before the larger animals undergo sufficient reduction to reach the 

 death point, but this occurs only where the differences are extreme. 



In general the indirect method is of value as a means of confirm- 

 ing the results of the direct method, and it can be applied to certain 

 forms where the direct method may be complicated by the relation 

 between surface and volume. The concentration to be used for 

 either method must of course be determined for each species. 



OTHER METHODS 



There are other physiological differences between young and 

 old organisms besides the rate of metabolism. In many cases 

 marked differences in motor activity exist between young and old 



1 Since I was unaware of this relation between the capacity for acclimation and 

 the nutritive condition at the time of my earlier experiments on rejuvenescence by 

 starvation, the use of the indirect method in those experiments led to incorrect con- 

 clusions concerning the changes in rate during starvation (Child, 'n, pp. 547-55), 

 but correction has been made in a later paper (Child, '140). The reader is also 

 referred to chapter vii below. 



