has brought us to that part of our problem that is concerned with the nature and origin of organic adaptations. Without tarrying to attempt a definition of adaptation I will only emphasize the fact that many of the great naturalists, from Aristotle onward, have recognized the purposeful or design-like quality of vital phenomena as their most essential and fundamental characteristic. Herbert Spencer defined life as the continuous _adjustment_ of internal relations to external relations. It is one of the best that has been given, though I am not sure that Professor Brooks has not improved upon it when he says that life is "response to the order of nature." This seems a long way from the definition of Verworn, heretofore cited, as the "metabolism of proteids." To this Brooks opposes the telling epigram: "The essence of life is not protoplasm but purpose." Without attempting adequately to illustrate the nature of organic adaptations, I will direct your attention to what seems to me one of their most striking features regarded from the mechanistic position. This is the fact that adaptations so often run counter to direct or obvious mechanical conditions. Nature is crammed with devices to protect and maintain the organism against the stress of the environment. Some of these are given in the obvious structure of the organism, such as the tendrils by means of which the climbing plant sustains itself against the action of gravity or the winds, the protective shell of the snail, the protective colors and shapes of animals, and the like. Any structural feature that is useful because of its construction is a structural adaptation; and when such adaptations are given the mechanist has for the most part a relatively easy task in his interpretation. He has a far more difficult knot to disentangle in the case of the so-called functional adaptations, where the organism modifies its activities (and often also its structure) in response to changed conditions. The nature of these phenomena may be illustrated by a few examples so chosen as to form a progressive series. If a spot on the skin be rubbed for some time the first result is a direct and obviously mechanical one; the skin is worn away. But if the rubbing be continued long enough, and is not too severe, an indirect effect is produced that is precisely the opposite of the initial direct one; the skin is replaced, becomes thicker than before, and a callus is produced that protects the spot from further injury. The healing of a wound involves a similar action. Again, remove one kidney or one lung and the remaining one will in time enlarge to assume, as far as it is able, the functions of both. If the leg of a salamander or a lobster be amputated, the wound not only heals but a new leg is regenerated in place of that which has been lost. If a flatworm be cut in two, the front piece grows out a new tail, the hind piece a new head, and two perfect worms result. Finally, it has been