without the aid of this senescent interpretation, if I may so term it, would be utterly impossible. This is an enticing subject, and I wish I had both time and competency to dwell upon it. But it is aside, as you see, from the main inquiries with which we have been occupied, for our inquiries concern chiefly the effect of cell-change upon the properties of the body, and the correlation of cell-change with age.
A natural branch of our topic is, however, that of longevity, the duration of life. Concerning this, we have very little that is scientifically satisfactory that we can present. Ye know, of course, as a fundamental principle, that every animal must live long enough to reproduce its kind. Did that not occur, the species would of course become extinct, and the mere fact that the species is existing proves, of course, this simple fact—that life has lasted long enough for the parents to produce offspring. The consideration of this fact has led certain naturalists to the supposition that reproduction is the cause of their termination of life; but it is not, it seems to me, at all to be so interpreted. "We know, in a general way, that large animals live longer than small ones. The elephant is longer lived than the horse, the horse than the mouse, the whale than the fish, the fish than the insect, and so on through innumerable other instances. At first this seems a promising clue, but if we think a moment longer we recognize quickly the fact that a parrot, which is much smaller than a dog, may live one hundred years, whereas a dog is very old at twenty. There are insects which live for many years, like the seventeen-year locusts, and others which live but a single year or a fraction even of one year, and yet the long-lived and the short-lived may be of the same size. It is evident, therefore, that size is not in itself properly a measure of the length of life. Another supposition, which at first sounds very attractive, is that which explains the duration of life by the rate of wear, of the using up, of the wearing out, of the body. This theory has been particularly put forward by Professor Weismann, who in his writings calls it the Abnutzungstheorie—the theory of the wearing out of the body. But the body does not really wear out in that sense. It goes on performing the functions for a long time, and after each function is performed the body is restored, and we do not find at death that the parts have worn out. But, as we have seen, we do find at death that there has been an extensive cytomorphosis, cell-change, and that the living material, after having acquired its differentiation, passes now in one part, now in another, then in a third, to a yet further stage, that of degeneration, and the result of degeneration, or atrophy, as the case may be, is that the living protoplasm loses its living quality and becomes dead material, and necessarily the functional activity ceases. We must, it seems to me, conclude that longevity, the duration of life, depends upon the rate of cytomorphosis. If that cytomorphosis is
