in the rate of growth which will perhaps help to illustrate it; and in the next of our pictures we see this other form of representation before us. This vertical line represents the length of time which it takes a young male guinea-pig to add ten per cent, to its weight the first time. Here the third time—the fourth—the fifth—and you see as it is growing older and older it takes the animal longer and longer to add ten per cent, to its weight. Finally we get to the nineteenth addition, and we see that the period is very long indeed. How long that period is we can judge by the figures here upon the left, which represent the length of the days. From the base line to this one marked "ten" is a period of ten days, and you see from the time the guinea-pig has added to its weight ten per cent, for the nineteenth time it does it so slowly that it requires ten days and more; for the twenty-first time, nearly twenty; for the twenty-second time, nearly forty. Here where the number of observations becomes small, the curve grows very irregular. Thus we demonstrate that as the animal grows older it takes longer and longer to add ten per cent, to its weight. In the other sex, as the next slide shows, the same phenomena can be clearly demonstrated; here are the periods as before, lengthening out, as you see, at first; then becoming very long indeed. In the following slide I have another
