228 AGE, GROWTH, AND DEATH 



in that sense. It goes on performing the functions 

 for a long time, and after each function is performed 

 the body is restored, and we do not find at death that 

 the parts have worn out. But, as we have seen, we 

 do find at death that there has been an extensive 

 cytomorphosis, cell-change, and that the living ma- 

 terial, after having acquired its differentiation, passes 

 now in one part, now in another, then in a third, to a 

 yet further stage, that of degeneration, and the result 

 of degeneration, or atrophy, as the case may be, is 

 that the living protoplasm loses its living quality and 

 becomes dead material, and necessarily the functional 

 activity ceases. We must, it seems to me, conclude 

 that longevity, the duration of life, depends upon the 

 rate of cytomorphosis. If that cytomorphosis is rapid, 

 the fatal condition is reached soon, if it is slow, the 

 fatal condition is postponed. And cytomorphosis in 

 various species and kinds of animals must proceed 

 at different rates and at different speeds at different 

 ages. Birds grow up rapidly during their period of 

 development ; the cell-change occurs at a high speed, 

 far higher than that which occurs in man, probably, 

 during his period of development. But after the bird 

 has acquired its mature development, it goes on almost 

 upon a level for a long time ; the bird which becomes 

 mature in a single year may live for a hundred or 

 even more. There can be during these hundred years 

 but a very slow rate of change. But in a mammal, a 

 dog or a cat, creatures of about the same bulk as 

 some large birds, we find that the early development is 



