Life is unknown to us except in association with bodies: we only know living bodies and call them organisms. It is the final object of all biology to tell us what it ultimately means to say that a body is “living,” and in what sorts of relation body and life stand one to the other. But at present it is enough to understand the terms “body” and “living” in the ordinary and popular sense. Regarding living bodies in this unpretentious manner, and recollecting what the principal characters are of all bodies we know as living ones, we easily find that there are three features which are never wanting wherever life in bodies occurs. All living bodies are specific as to form--they “have” a specific form, as we are accustomed to say. All living bodies also exhibit metabolism; that is to say, they stand in a relation of interchange of materials with the surrounding medium, they take in and give out materials, but their form can remain unchanged during these processes. And, in the last place, we can say that all living bodies move; though this faculty is more commonly known among animals only, even elementary science teaches the student that it also belongs to plants. Therefore we may ask for “laws of nature” in biology about form, about metabolism, and about movements. In fact, it is according to this scheme that we shall arrange the materials of the biological part of our lectures, though, as we cannot regard the three divisions as equally important in their bearing on our ultimate purposes, we shall not treat them quite on equal terms. It will appear that, at least in the present state of science, the problems of organic form and of organic movement have come into much closer relation to philosophical analysis than have most of the empirical data on metabolism.