126 THE RATE OF GROWTH [ch. 



inequality of age; for in a population only a few weeks old, a few 

 days sooner or later in the date of birth would make more diiference 

 than later on. But a more important matter, to be seen in man- 

 kind (Fig. 22), is that variability of stature runs j)ari passu, or 

 nearly so, with the rate of growth, or curve of annual increments 

 (cf. Fig. 12). The curve of variability descends when the growth- 

 rate slackens, and rises high when in late boyhood growth is speeded 

 up. In short, the amount of variability in stature or in weight is 

 correlated with, or is a function of, the rate of growth in these 

 magnitudes. 



Judging from the evidence at hand, we may say that variabihty 

 reaches its height in man about the age of thirteen or fourteen, 

 rather earlier in the girls than in the boys, and rather earher in the 

 case of stature than of weight. The difference in this respect between 

 the boys and the girls is now on one side, now on the other. In 

 infancy variabihty is greater in the girls; the boys shew it the 

 more at five or six years old; about ten years old the girls have 

 it again. From twelve to sixteen the boys are much the more 

 variable, but by seventeen the balance has swung the other way 

 (Fig. 23). 



Coefficient of variability (ojM x 100) in man, at various ages 

 Age ... o 6 7 8 9 10 11 12 13 14 15 16 17 18 



ytature 



I 



Mean — 10-3 — 111 — 11-5 — 11-9 14-8 15-7 13-5 11-9 — -- 



