62 THE RATE OF GROWTH [ch. 



but a very rough preliminary statement, and that the boy grew 

 quickly during some, and slowly during other, of his twenty years. 

 It becomes necessary therefore to study the phenomenon of growth 

 in successive small portions ; to study, that is to say, the successive 

 lengths, or the successive small differences, or increments, of 

 length (or of weight, etc.), attained in successive short increments 

 of time. This we do in the first instance in the usual way, by 

 the "graphic method" of plotting length against time, and so con- 

 structing our "curve of growth." Our curve of growth, whether 

 of weight or length (Fig. 3), has always a certain characteristic 

 form, or characteristic curvature. This is our immediate proof of 

 the fact that the rate of growth changes as time goes on ; for had 

 it not been so, had an equal increment of length been added in 

 each equal interval of time, our "curve" would have appeared 

 as a straight line. Such as it is, it tells us not only that the rate 

 of growth tends to alter, but that it alters in a definite and orderly 

 way ; for, subject to various minor interruptions, due to secondary 

 causes, our curves of growth are, on the whole, "smooth" curves. 



The curve of growth for length or stature in man indicates 

 a rapid increase at the outset, that is to say during the quick 

 growth of babyhood ; a long period of slower, but still rapid and 

 almost steady growth in early boyhood; as a rule a marked 

 quickening soon after the boy is in his teens, when he comes to 

 "the growing age" ; and finally a gradual arrest of growth as the 

 boy "comes to his full height," and reaches manhood. 



If we carried the curve further, we should see a very curious 

 thing. We should see that a man's full stature endures but for 

 a spell; long before fifty* it has begun to abate, by sixty it is 

 notably lessened, in extreme old age the old man's frame is 

 shrunken and it is but a memory that "he once was tall." We 

 have already seen, and here we see again, that growth may have 

 a "negative value." The phenomenon of negative growth in old 

 age extends to weight also, and is evidently largely chemical in 

 origin : the organism can no longer add new material to its fabric 

 fast enough to keep pace with the wastage of time. Our curve 



* Dr Johnson was not far wrong in saying that "life decHnes from thirty-five" ; 

 though the Autocrat of the Breakfast-table, like Cicero, declares that "the furnace 

 is in full blast for ten years longer." 



