Page:Popular Science Monthly Volume 71.djvu/203

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question whether a baby grew an ounce in a certain length of time, and a boy a pound in the same time, for the pound might not be the same percentage of advance to the boy that the ounce would be to the baby. In reality with an advance of an ounce the baby might be growing faster than the older boy with the addition of the pound.

In the next slide which we are to have thrown upon the screen we have my method of measuring rate of growth illustrated graphically. There is here a curve which represents the rate of growth of male guinea-pigs. The figures at the bottom indicate the age of the animals in days. When guinea-pigs are born, they are very far advanced in development, and the act of birth seems to be a physiological

PSM V71 D203 Graph of the daily percentage of weight increments of male guinea pigs.png

Fig. 21. Curve showing the Daily Percentage Increments in Weight of Male Guinea Pigs.

shock from which the organism suffers, and there is a lessening of the power of growth immediately after birth. But in two or three days the young are fully recovered, and after that restoration they can add over five per cent, to their weight in a single day. But by the time they are 17 days old, as represented by this line, they can add only four per cent., and by the time they are 24 days old, less than two per cent.; at 45 barely over one per cent.; at 70 still over one per cent.; at 90 less; at 160 less; and towards the end the curve continues dropping off, coming gradually nearer and nearer to zero, to which it closely approximates at the age of 240 days. In about a year, the guinea-pig attains nearly its full size. You notice that this curve is somewhat irregular. Such is very apt to be the result from statistics when the number of observations is not very large. It means simply that there was not a sufficiently large number of animals measured to give an absolutely even and regular set of averages. But the general course of the curve is very instructive. In the earlier condition of the young guinea-pig there is a rapid decline; in the later, a slow decline.