It was apparently from a feeling that the velocity of growth ought in some way to be equated with the mass of the growing structure that Minot[102] introduced a curious, and (as it seems to me) an unhappy method of representing growth, in the form of what he called “percentage-curves”; a method which has been followed by a number of other writers and experimenters. Minot’s method was to deal, not with the actual increments added in successive periods, such as years or days, but with these increments represented as _percentages_ of the amount which had been reached at the end of the former period. For instance, taking Quetelet’s values for the height in centimetres of a male infant from birth to four years old, as follows: Years 0 1 2 3 4 cm. 50·0 69·8 79·1 86·4 92·7 Minot would state the percentage growth in each of the four annual periods at 39·6, 13·3, 9·6 and 7·3 per cent. respectively. Now when we plot actual length against time, we have a perfectly definite thing. When we differentiate this _L_/_T_, we have _dL_/_dT_, which is (of course) velocity; and from this, by a second differentiation, we obtain _d_^2 _L_/_dT_^2, that is to say, the acceleration. {73} But when you take percentages of _y_, you are determining _dy_/_y_, and when you plot this against _dx_, you have (_dy_/_y_)/_dx_, or _dy_/(_y_ ⋅ _dx_), or (1/_y_) ⋅ (_dy_/_dx_), that is to say, you are multiplying the thing you wish to represent by another quantity which is itself continually varying; and the result is that you are dealing with something very much less easily grasped by the mind than the original factors. Professor Minot is, of course, dealing with a perfectly legitimate function of _x_ and _y_; and his method is practically tantamount to plotting log _y_ against _x_, that is to say, the logarithm of the increment against the time. This could only be defended and justified if it led to some simple result, for instance if it gave us a straight line, or some other simpler curve than our usual curves of growth. As a matter of fact, it is manifest that it does nothing of the kind. _Pre-natal and post-natal growth._