THE PROBLEM AND METHODS OF INVESTIGATION 65 



young or old, not merely morphologically but physiologically. We 

 can of course distinguish embryonic, larval, and juvenile forms from 

 adults by their morphological characters, and in many cases by 

 their physiological characters as well, but it is not always easy to 

 distinguish younger and older individuals of the same general stage 

 of the life cycle. In the higher animals certain morphological 

 changes which are apparently characteristic of senescence have 

 been observed in some cells, but the morphological features of the 

 cells of different organisms are so different and the visible changes 

 so slight in many cases that, though it is usually possible to dis- 

 tinguish embryonic from definitely differentiated cells, it is very 

 often impossible to distinguish old and young individuals of the 

 same general stage by the morphological characters of their cells. 

 Measurements of the metabolism or of the rate of growth in man 

 and the mammals show that the rates of both per unit of weight 

 decrease as age advances, but the methods employed for such forms 

 are not readily applicable in many other cases, because of the con- 

 ditions of existence, the small size, the low rate of metabolism, 

 etc. In the course of my investigation of the process of reproduc- 

 tion in the lower invertebrates a method based on the physiological 

 resistance or susceptibility of the animals to certain conditions has 

 been developed, which has proved to be of great value in distin- 

 guishing physiologically young from old organisms as well as for 

 various other purposes. 



SUSCEPTIBILITY IN RELATION TO KATE OF METABOLISM 



It is a familiar fact that the susceptibility or physiological 

 resistance of man and the higher animals to various external factors, 

 and particularly to those which depress, changes with advancing 

 age, and I have found that this is also true for the lower animals, as 

 far as they have been tested. On the basis of this relation between 

 susceptibility and physiological age, it has been possible to develop 

 a method which not only enables us to distinguish differences in 

 age, but affords a means of comparing in a general way the rates 

 of the metabolic processes, or of certain fundamental metabolic 

 reactions in different animals. This method, which may be called 

 the susceptibility, physiological resistance, or survival- time method, 



