Page:Popular Science Monthly Volume 71.djvu/206

200 form of representation of this same phenomenon as it occurs in the human subject. Here is a diagram of growth, which represents, as accurately as I could determine it, the curve complete for man from the date of birth up to the age of forty years. It has been calculated by a simple mathematical process where these ten-per-cent. increments fall, and from each point in this curve where there has been such an increment, a vertical line has been drawn, as you see here. These lines are very close together at the start. One ten per cent, after another follows in a short interval of time, but gradually the time, as indicated by the space between two of these vertical lines, increases, and when the individual is three years old, you can see there has been a very great

lengthening out of the period which is necessary for it to add ten per cent, to its weight. Then it comes at the age of twelve to a period of slightly more rapid growth, a fluctuation which is characteristic of man, but does not appear in the majority of animals. After that comes very rapidly the enormous lengthening of the period; and I have not added the last ten per cent, because the curve here at the top, you see, is not very regular, and it could not be calculated with certainty. Our diagram is merely another form of graphic representation of the fact that the older we are the longer it takes us to grow a definite proportional amount.

The next slide carries us into another part of our study, away from the mammals which we have thus far considered, into the class of birds. The growth of chickens is represented here. Now a chicken is born in a less matured state than a guinea-pig, and has a good deal