to us two kinds of spectacles. Sometimes we see that the cells causing the diseased condition are more or less of the sort which naturally belong in the body; that they are present where they do not belong, or they are present where they ought to be, but in excessive quantity. There is a kind of tumor which we call a bony tumor. It consists of bone cells such as are naturally present in the body, but that which makes this growth of bone a tumor is its abnormal dimensions, or perhaps its being altogether in the wrong place. The second sort of pathological alteration, which I had in mind, is that in which the cells really change their character. Now, the young cells are those which can change most; in which the genetic restriction has least come into play; and accordingly we find that a large number of dangerous, morbid growths, tumors, arise from cells of the young type, and these cells, having an extreme power of multiplication, grow rapidly, and they may assume a special character of their own; their genetic restriction has not gone so far that all their possibilities of change in the way of differentiation have been fixed; there is a certain range of possibilities still open to them, and they may turn in one direction or the other. Hence there may be pathological growths of a character not normally present in the body. It seems to me, so far as my knowledge of this subject enables me to judge, to be true that all such pathological growths depend upon the presence of comparatively young and undifferentiated cells being turned into a new direction. The problem of normal development and of abnormal structure is one and the same. Both the embryologist and the anatomist, on the one hand, and the pathologist and the clinician on the other, deal ever with these questions of differentiation, and practically with no others. All that occurs in the body is the result of various differentiations, and whether we call the state of that body normal or pathological matters little; still the cause of it is the differentiation of the parts.
The second of the collateral topics which I should like briefly to allude to is another branch of the study of senescence. The fact was first emphasized by the late Professor Alpheus Hyatt that in many animals there exist parts formed in an early stage and thereafter never lost. The chambered nautilus is an animal of this kind. The innermost chamber represents the youngest shell of the nautilus, and as its age increases, it forms a new chamber in its shell, and so yet more and more until the coil is complete. When we examine a shell of that kind we see permanently before us the various stages, both young and old, as recorded in shell formation. And so too in the sea-urchin, and in many of the common shell-fish, we find the double record, of youth and old age, preserved permanently. This has made it possible for Professor Hyatt and for Professor Robert T. Jackson, who has adopted a similar guiding principle, to bring a great deal of new light into the study of animal changes, and to attack the solution of problems which