AGE, GROWTH AND DEATH 493 



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 dis- 

 cover 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 tbe 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 



