CHANGES IN METABOLISM WITH AGE. 127 



Practically the linear nature of the change of metabolism with age 

 is of great importance in connection with the establishment of standard 

 control series to be used in applied calorimetry a subject to be fully 

 discussed in Chapter VIII. 



For the purposes of throwing some light on the general problem of 

 senescence, we have brought together for comparison such quantita- 

 tive data as are available on the changes of another physiological 

 character with age. 



Pulse-rate in our own data shows a slight decrease with increasing 

 age. The amount of change is so small that its nature has not been 

 investigated. 



Referring to the problem of senescence, rejuvenescence, and death 

 in man and other higher animals, Child 14 says : 



"As regards the relation between senescence, death, and rejuvenescence, 

 the higher animals and man differ from the lower organisms in the limitation 

 of the capacity for regression and rejuvenescence under the usual conditions. 

 Senescence is therefore more continuous than in the lower forms and results in 

 death, which is the final stage of progressive development. These character- 

 istics of man and the higher animals are connected with the evolutionary 

 increase in the physiological stability of the protoplasmic substratum and the 

 higher degree of individuation which results from it." 



Now, without passing any judgment on the validity of Child's 

 extension to the higher vertebrates of his remarkable experimental 

 results with planarians and other lower forms, we may point out that 

 our own quantitative results fully substantiate his conclusion concern- 

 ing the greater continuity of senescence in the higher forms. In man, 

 changes in metabolism after physical maturity are not merely contin- 

 uous, they are uniform in amount, so that they can be reasonably well 

 represented by the slope of a straight line. 



14 Child, Senescence and Rejuvenescence, Chicago, 1915, p. 399. 



15 Italics ours. 



