time. This we do in the first instance in the usual way, by the ãgraphic methodã of plotting length against time, and so constructing our ãcurve of growth.ã Our curve of growth, whether of weight or length (Fig. 3), has always a certain charôÙacôÙterôÙisôÙtic form, or charôÙacôÙterôÙisôÙtic 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 ãcurveã 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, ãsmoothã 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 ãthe growing ageã; and finally a gradual arrest of growth as the boy ãcomes to his full height,ã and reaches manhood.
If we carried the curve further, we should see a very curious thing. We should see that a manãs full stature endures but for a spell; long before fifty17 it has begun to abate, by sixty it is notably lessened, in extreme old age the old manãs frame is shrunken and it is but a memory that ãhe once was tall.ã We have already seen, and here we see again, that growth may have a ãnegative value.ã 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