GROWTH AND SENESCENCE 



47 



YpsB 



Age -\feaps (Man), Days on-osophila) 



t> 30 30 -4Q 5Q 6Q TO 



90 Pneumonia 



eo l>iar>phea& Entente 



Pays B IP 20 90 40 



50 



eo to eo 90 jpo pposopriaag 



£8 



Days B 10 20 30 40 50 60 70 



eo 00 lOODP osophila * 



.001 

 .0006. 



k(t-W scale 



Figure 28. In this chart the mortality curves of man and fly (Figures 26 and 27) were 



made to coincide. 



in the fruit fly Drosophila at about ten days of age j in the dairy 

 cow at about seven years; in the domestic fowl at about one 

 year. The mathematical equation of this statement of the course 

 of declining vitality with age is 



V = Ae" kt 



S = Ae" (0 



in which V is the index of vitality, S is the index of senescence, 

 t the age, e the natural base of logarithms, and iook is the 

 percentage rate. This is also the equation used to represent the 

 decline in the velocity of a chemical reaction with time, which 

 is in harmony with the idea previously expressed that the course 

 of senescence may be limited by a chemical process. 



Some indices of senescence, such as the declining ability of 

 the eye to accommodate itself to near objects (cf. Figure 29), 

 do not follow the relationship represented by equation (1). 

 This apparent exception may be due to insufficient data. 



