ioo AGE, GROWTH, AND DEATH 



ten per cent, to its weight. Then it comes at the 

 age of twelve to a period of slightly more rapid 

 growth, a fluctuation which is characteristic of man, 

 but does not appear in the majority of animals. After 

 that comes very rapidly the enormous lengthening of 

 the period. I have not added the last ten per cent, 

 because the curve here at the top, you see, is not 

 very regular, and it could not be calculated with cer. 

 tainty. Our diagram is merely another form of 

 graphic representation of the fact that the older we 

 are the longer it takes us to grow a definite propor- 

 tional amount. 



Figure 33 carries us into another part of our 

 study, away from the mammals which we have thus 

 far considered, into the class of birds. The growth 

 of chickens is represented here. Now a chicken is 

 born in a less matured state than a guinea-pig, and 

 has a good deal higher efficiency of growth at first* 

 In a chicken, as in a guinea-pig, birth is a disturbing 

 factor, and growth immediately after the hatching of 

 the chicken is a little impeded, but the chick quickly 

 recovers and, as we see, the first time when the rate 

 can be distinctly measured we get a nine-per-cent. 

 addition to the weight in a single day. In a chicken, 

 as in the guinea-pig, the rate gradually diminishes. 

 The change from the rapid decline at .first to the 

 later slower decline is more gradual ; the curve is 

 more distinctly marked in the chicken as a round 

 curve. There is not in the bird so distinct a separa- 

 tion of the preliminary rapid decline and the later 



