Do we not see, in fact, that the new ideas are indeed for the most part the ideas of young people. As Dr. Osier, in that much-discussed remark of his, has said, the man of forty years is seldom the productive man. Dr. Osier also mentioned the amiable suggestion of Trollope in regard to men of sixty, which has been so extremely misrepresented in the newspaper discussions throughout the country, causing biologists much amusement. But I think that Dr. Osier probably took a far too amiable view of mankind, and that in reality the period when the learning power is nearly obliterated is reached in most individuals very much earlier. As in every class of biological facts, there is here the principle of variation to be kept in mind. Men are not alike. The great majority of men lose the power of learning, doubtless some more and some less, we will say, at twenty-five years. Few men after twenty-five are able to learn much. They become day laborers, mechanics, clerks of a mechanical order. Others probably can go on somewhat longer, and obtain higher positions; and there are men who, with extreme variations in endowment, preserve the power of active and original thought far on into life. These of course are the exceptional men, the great men.
We have lingered so long together studying phenomena of growth, that it is natural to allude to one more, which is as singular as it is interesting, namely, the increase in size of Americans. It was first demonstrated by Dr. Benjamin A. Gould in his volume of statistics derived from the records of the Sanitary Commission—a volume which still remains the classic and model of anthropometric research. Any one, however, can observe that the younger generation of to-day tends conspicuously to surpass its parents in stature and physical development. How to explain the remarkable improvement we do not know. Our discovery of the fact that the very earliest growth is so enormously rapid, makes that earliest period especially important. If the initial growth can be favored a better subsequent development presumably would result. In brief, I find myself led to the hypothesis that the better health of the mothers secures improved nourishment in the early stages of the offspring, and that the maternal vigor is at least one important immediate cause of the physical betterment of the children. Much is said about the degeneracy of the American race, but the contrary is true—the American race surpasses its European congeners in physical development.
You will naturally wish to ask, before I close the series of lectures, two questions. One, how can rejuvenation be improved; the other, how can senescence be delayed. These questions vriW strike every one as very practical. But the first, I fear, is not an immediately practical question, but rather of scientific interest, for we must admit that the production of young individuals is, on the whole, very well accomplished and much to our satisfaction. But in regard to growing old.