of the brain, not only a diminution of the organ as a whole, but there is the further degradation which consists in the yielding of the nobler to the baser part, if I may so express myself. That, you recognize, necessarily implies a loss of function. The brain can not under senile conditions do the sort of fine and efficient work which it could do before. Now if we go on from insects to yet lower organisms, we see less and less appearing of an advance in organization, of correlated loss of parts, and when we get far enough down in the scale, senescence becomes very vague. The change from youth to old age in a coral or in a sponge is at best an indefinite matter.
I should like, did the length of the course permit, to enlarge greatly upon this aspect of the question, and explain to you how it is that as the organism rises higher and higher in the scale, old age becomes more and more marked, and in no animal is old age perhaps so marked, certainly in no animal is it more marked, than in ourselves. The human species stands at the top of the scale and it also suffers most from old age. We shall learn, I hope, more clearly later on in the course of these lectures, that this fact has a deeper significance, that the connection between old age and advance in organization, advance in anatomical structure, is indeed very close, and that they are related to one another somewhat in fashion of cause and effect; just how far each is a cause and how far each is an effect it would perhaps be premature to state very positively; but I shall show you, I think in a convincing way, that the development of the anatomical quality, or in other words of what we call organic structure, is the fundamental thing in the investigation of the processes of life in relation to age. We can see it illustrated again very clearly indeed when we turn to the study of plant life, for plants also grow old. Take a leaf in the spring. It is soft as the bud opens. The young leaf is delicate. It has a considerable power of growth. It expands freely, and soon becomes a leaf of full size. Then comes the further change by which the leaf gets a firmer texture; the production of anatomical quality in the leaf, so to speak, goes on through the summer, and the result of that advance in the anatomical quality is that the delicate, youthful softness and activity of the leaf is stopped. It can not grow any more; it can not function as a leaf properly any more. The development of its structure has gone too far and the leaf falls and is lost, and must be replaced by a new leaf the next year. When we examine the changes that go on in any flowering plant, we observe always that there is this production of structure, and then the decay, the end or death. At first structure comes as a helpful thing, increasing the usefulness of the part, and then it goes on too far and impairs the usefulness, and at last a stage is produced in which no use is possible any longer—the thing is worthless. It is cast away in the case of the plant life; and this casting away of the useless is a thing not by any
