life in our demands upon our organism, and still find that our body, is capable of making the necessary response. Ordinarily the amount of blood which we require is moderate in amount—moderate in the sense that the destruction of the blood continually going on in the body is not a very rapid process; but if, through some accident, a person loses a large quantity of blood then by one of these teleological reactions of which I have spoken, the production of new blood is increased, the loss is soon made up, and we discover that the blood, so to speak, has been repaired. Or when a little of the skin is lost, it quickly heals over. That again is due to the power of repair. Ordinarily so long as the skin remains whole that power is not called into action, but if a wound comes, then the regenerative force resident always in the skin, but inactive, comes into play and produces the mending which is such a comfort. So in old people, some of this luxury of reparative power persists, so that they can recover from wounds in a far better way than we should imagine if we judged them only by the general physiological and anatomical decline exhibited throughout all parts of the body. Some of the luxury of repair comes in usefully in old age. Now if we consider all these changes in the most general manner, we perceive that they are clearly of one general character; they imply an alteration in the anatomical condition of the parts; but it is an alteration which does not differ fundamentally in kind from the alterations which have gone on before, but it does differ in the extent and in part in the degree to which these alterations have taken place. When the elastic cartilaginous rib becomes bony, nothing different is happening from that which happened before, for there was a stage of development when the entire rib consisted of cartilage, and in the progress of development toward the adult condition that cartilage was changed gradually into bone, thus producing the characteristic, normal, efficient bony rib of the adult. When old age intervenes, the change of the cartilage into bone goes yet further, but it progresses in such a way that it is no longer favorable, but unfavorable. We have then in this case a clear illustration of a principle of change in the very old which is, I take it, perhaps sufficiently well expressed by saying that the change which is natural in the younger stage is in the old carried to excess. But there is in addition to this, something more, of which I have already spoken, namely the atrophy of parts, and by atrophy we mean the diminution, the lessening of the volume of the part. There is a partial atrophy of the brain in consequence of which that organ becomes smaller; there is an extensive atrophy of the muscles in consequence of which their volume is diminished, and their efficiency decreased. Atrophy is preeminently characteristic of the very old, and we see in very old persons that it becomes each year more and more pronounced. Indeed, it has been said recently by Professor Metchnikoff, a distinguished Russian zoologist, now connected with the
