tions as to what is essential in the change from youth to old age, and that in consequence of these generalizations, now possible to us, new problems present themselves to our minds, which we hope really to be able to solve, and that in the solving of them we shall gain a sort of knowledge, which is likely to be not only highly interesting to the scientific biologist, but also to prove, in the end, of great practical value. Surely we can not hope to obtain any power over age, any power over the changes which the years bring to each of us, unless we understand clearly, positively and certainly, what these changes really are. I think you will learn, if you do me the honor to follow the lectures further, that the changes are indeed very different from what we should expect when we start out on a study of age, and that the contributions of science in this direction are novel and to some degree startling. We can begin to approach this broader view of our subject if we pass beyond the consideration of man.
If we turn from man to the animals which we are most familiar with, the common domestic quadrupeds, we see that they undergo a series of changes not very dissimilar to those which man himself must pass through. An old horse, an old dog, an old cat, shows pretty much the same sort of decrepitudes which characterize old men. But when we pass farther down in the scale to the fishes, or even to a frog, we discover great differences. Do you think you could tell a frog when it is old by the way it walks—for it never walks—or a fish by the amount of hardening of the lungs, when it has none? Yet the lack of lungs is characteristic of the fish. And what becomes of the theory of arterial sclerosis when we go still lower in the animal kingdom, towards its lowermost members, and find creatures which live and thrive and have lived and thriven for countless generations, yet have no arteries at all? They, of course, do not grow old by any change of their arteries. But when we come to study these various animals more carefully, we learn that in them the anatomical and physiological features which I have indicated to you in my description of the changes in the human being, are paralleled, as it were, by similar changes; but only by similar, not by identical, changes. If we examine the insects, for instance, we see that in an old insect there is a hardening of the outer crust of the body which serves as a shell and a skeleton at once. That hardening increases with the age of the individual. We can see in the insect a lessening development of the digestive tract, and we can see—it has been demonstrated with particular nicety—a degradation of the brain. Insects have a very small brain, but when a bumblebee, or a honeybee, grows old, as he does in a few weeks after he acquires his wings, we see that the brain actually becomes smaller, and not only that, but as I shall be able to demonstrate to you with the lantern in the next lecture, the elements which build up the brain have each of them become smaller and the diminution in the size of
