Page:Popular Science Monthly Volume 71.djvu/211

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of animal indicated at the left to double its weight after birth. A man requires 180 days to double his weight; a horse, 60; a cow, 47; a goat, 19; a pig, 18; a sheep, 10; a cat, 9; a dog, 8; a rabbit, 6 (or possibly 7 days). Now here are analyses of the milk. The main point of interest is to be found in the figures in this column, which represent the amount of albuminoid, or proteid material contained in the milk. You will observe that for man the proportion is lowest, 1.6 per hundred parts; the horse has a little more—2; cattle—3.5; and so the values run. In other words, it is obvious that the less the proteid in the milk, the longer does the species require to double its weight. This looks at first sight as if there were a relation between the composition of the milk and the period of growth of the animal; but you know very well that if you take the milk of a cow, which is very much richer in proteid material, and feed it to a baby, a human baby, that baby does not grow at the same rate as the young cow, but grows at the human rate. It is obvious, therefore, that it is somewhat more complicated than a mere question of food supply. We have in fact one of the beautiful illustrations of the teleological mechanism of the body. These various species have their characteristic rates of growth, and by an exquisite adaptation, the composition of the mother's milk has become such that it supplies the young of the species each with the proper quantum of proteid material which is needed for the rate of growth that the young offspring is capable of. It is a beautiful adjustment, but there is not a causal relation between proteid matter and this rate of growth. It is an example of correlation, not of causation.

We pass now to the next of our slides, which carries us over into the study of our own species. It is not possible at the present time to represent in any form of curve which I have seen the daily percentages of increment for man covering the whole period of growth. In order to get the results together, I have confined myself here to the representation of the yearly percentages. Now from the age of zero to the age of one year, you see according to this table a child is able to increase its weight 200 per cent. But from the beginning of the first to