GROWTH AND SENESCENCE 



63 



Yw 

 70 



iyl!at 



stop to discuss this problem. We shall only express the general 

 theory of this phenomenon and substantiate this theory by 

 means of a curve (Figure 43). 



It was pointed out in the in- 

 troductory section that growth in 

 higher animals is due primarily 

 to the increasing number of cells ; 

 also, that the successive increases 

 in the number of cells decline 

 with age due to some unfavor- 

 able physico-chemical influence, 

 or changes in its "universe," or 

 in the medium surrounding the 

 cells. (It is on the basis of the 

 assumption that such a physico- 

 chemical change takes place that 

 we have employed the equation 

 of physico-chemical reactions to 

 represent the course of growth.) 

 As the division-rate of the cells 

 decreases with age, the cells 

 probably undergo some change 

 in their colloid structure. This 

 change probably constitutes se- 

 nescence, and death is due to the 

 breakdown of some essential or- 

 gan whose cells, weakened by the 



process of senescence, give way under a given set of unfavorable 

 conditions. 



Now, if there is a relation between the decline in the division 

 rate of cells with age (i.e., the decline in the speed of growth 

 with age) and duration of life as explained above, then the 

 average duration of life of a species of animals should be (all 

 other conditions being the same) inversely proportional to the 

 decline in the velocity of growth with age. As a matter of fact, 



Value of fc. 



Figure 43. The relation between the 

 rapidity with which the mature weight 

 is reached, k of equation (2), and dura- 

 tion of life. The smooth curve was 

 drawn on the assumptions that (1) the 

 duration of life is inversely proportional 

 to the value of k, that is, it is roughly 

 proportional to the time required to 

 reach maturity, and (2) that the dura- 

 tion of life of Donaldson's female rat 

 is three years. 



