96 



THE RATE OF GROWTH 



[CH. 



We have now obtained two different but closely related curves, 

 based on the selfsame facts or observations, and illustrating them 

 in different ways. One is the inverse of the other ; one is the integral 

 and one the differential of the other; and each makes clear to the 

 eye phenomena which are imphcit, but are less conspicuous, in the 

 other. We are using mathematical terms to describe or designate 

 them; but these "curves of growth" are more comphcated than 

 the curves with which mathematicians are wont to deal. In our 

 study of growth we may well hope to find curves simpler than these; 



20» 



3 5 7 9 II 13 15 17 19 21 

 Age in years 

 Fig. 5. Annual increments of growth in man. From Quetelet's Belgian data. 



but in the successive annual increments of a boy's growth (as Fig. 5 

 exhibits them) we are deahng with no one continuous operation 

 (such as a mathematical formula might define), but with a succession 

 of events, changing as times and circumstances change. 



Our curve of increments, or of first differences, for man's stature 

 (Fig. 5) is based, perforce, on annual measurements, and growth 

 alters quickly enough at certain ages to make annual intervals unduly 

 long; nevertheless our curve shews several important things. It 

 suffices to shew, for length or stature, that the growth-rate in early 

 infancy is such as is never afterwards re-attained. From this high 

 early velocity the rate on the whole falls away, until growth itself 



