GROWTH AND SENESCENCE 45 



year to 63 in the eighth year. The smooth curve was drawn on 

 the assumption that the egg production declines by 12.3 per 

 cent per year; that is, that each year's production is 87.7 per 

 cent of the production during the preceding year. As far as this 

 particular group of birds is concerned, it is evident that the law 

 of senescence as measured by the decline in egg production is 

 that the increase in senescence, or decline in vitality, takes place 

 at a constant rate, namely, at a decline of 12.3 per cent per year. 

 The data on which Figure 25 is based are given in Table I. 



Table I 



THE DECLINE IN EGG PRODUCTION WITH AGE IN THE 

 DOMESTIC FOWL 



Egg production per year. Nov. i to Nov. i 



Observed* Calculated^ 



158 158.5 



140 140. 1 



124 123-9 



110 109.6 



95 96.9 



89 85.7 



7i 75-7 



63 67.0 



Figures 16 and 27 indicate respectively, the course of mor- 

 tality in man due to different diseases, and the course of mor- 

 tality in a population of the fruit fly Drosophila. In these fig- 

 ures the scale of ordinates is divided in a geometrical progres- 

 sion and whenever a set of changing values falls on a straight 

 line on this paper, then it is obvious that the change takes place 

 in a geometrical progression, in other words, at a constant per- 

 centage rate — for, by definition, a geometrical progression is 



* The observed values are averages of two breeds and eight different groups 

 of birds kept under different conditions of management. To save space in this 

 general publication, the individual records, and their significance, are omitted. 



j Calculated from the equation y = i79.2e _012t in which y is the yearly egg 

 production at the age t. The egg production during any year is 88 per cent of 

 the preceding year's production. 



