44 SENESCENCE AND REJUVENESCENCE 



The organism does not construct itself for function as the vitalistic 

 and chemical theories maintain: it constructs itself by function. 

 When the supply of nutritive material from without is insuffi- 

 cient, the previously accumulated structural material may serve 

 as a source of energy to a much greater extent than when nutritive 

 material is present in excess, and under these conditions the new 

 structural material, if any is formed, may be insufficient to cover 

 the loss and reduction results. Such reduction may involve the 

 whole organism to a greater or less extent, as in the flatworms and 

 other simple animals, or it may involve only or chiefly certain parts, 

 but in all cases we find that some parts or substances are involved 

 to a greater extent than others. In a starving flatworm, for 

 example, certain organs may disappear entirely before death occurs, 

 while others retain more nearly their usual proportions. Much 

 has been made of this fact in a teleological sense (see, for example, 

 E. Schultz, '04), and it has been repeatedly pointed out that the 

 organs least affected are those most essential to the life of the 

 organism. But a teleological interpretation seems to be quite 

 unnecessary. In general it is very evidently the case that those 

 organs which are most constantly, most frequently, or most in- 

 tensely active in the life of the organism undergo least reduction 

 during starvation. There is some reason to believe that the 

 structural substratum of the cells of such organs is more stable than 

 that of cells which possess in general a low rate of metabolism. The 

 nervous system undergoes least reduction during starvation, and 

 during the earlier stages of development it certainly has the highest 

 metabolic rate of any part of the body, and in many cases, if not 

 in all, this condition persists throughout life. Furthermore, during 

 the later stages of life its special functional activity is certainly 

 almost if not quite continuous. In such organs energy must be 

 derived to a much greater extent from nutritive substances than 

 from the substratum of the cells itself. Consequently, during 

 starvation their losses are less and are more completely repaired 

 than in organs where the substratum is less stable. Thus the more 

 active and therefore the more persistent organs maintain them- 

 selves largely at the expense of other less active parts in which the 

 degradation of the structural substratum occurs more readily. 



