66 



THE EVOLUTION OF DEATH 



By the time they are seventeen days old, they grow only 

 only about 4 per cent, and at forty-five days only a little 

 more than i per cent, and from this age on the rate of growth 

 sinks slowly until at the end of the first year it becomes 

 almost zero. The general process is the same in females, 

 Fig. 28, as in males, although certain inequalities occur. 

 It is obvious that if we consider the curves, Figs. 27 and 28, 

 carefully, we can distinguish in them two chief periods, which, 

 however, pass into one another without definite boundaries. 



5 Tl 17 23 2933 tf 60 75 90 105 120 135 150 165 180 195 210 days 

 FIG. 28. Curve of the daily percentage increase in weight in female guinea-pigs. 



In the first, shorter period, the rate diminishes rapidly. This 

 period lasts about one and a half months. The second period 

 exhibits a much slower decrease and lasts perhaps ten months. 

 The result was unexpected. If we accept the rate of growth 

 as the measure of senescence, we must say that young animals 

 grow old enormously faster than old animals. Since alter- 

 ations in the rate of growth of guinea-pigs progress as de- 

 scribed, it was to be expected that in still younger stages of 

 development the rate of growth would be found still greater. 

 Now chickens when they enter the world are not so far 



