We ought now to consider rather closely--exhaustively we cannot--what is included under the term Biology. It stood for some time chiefly to mean an account of the structures of animals and plants, structure being pursued into ever further minuteness, down to the cell and the constituents and parts of the cell. With this has gone insistent inquiry into the process of reproduction and growth; and more lately, in bio-chemistry and bio-physics, the conformity of living substance to the order recognised in non-living matter has been, and is being, most eagerly investigated. And now a school of biologists is arising whose aim is the vindication of the claims of function as against the too exclusive study of structure. Function, of course, involves activity; and activity, in a complex, multicellular organism, involves the interplay of parts. This interplay, again, cannot be studied without reference to the environment, and to the relations between the organism in question and others--whether of its own or of other species. In this way it seems likely that biology--moving as it were in a spiral--will by-and-by return, though at a much higher level, to the standpoint of the older naturalists, whose interest in plants and animals was focussed more upon their activities, habits of life and special environment than upon their morphology--and even disdained not to consider their possible uses for man. Also, more thoroughly and extensively than before, the study of man himself is being caught up into the great web of Biology. It is seen as an integral part of Biology, and pursued in the biological spirit. Whether we look to psychology on the one hand, or to anthropology and its associated sciences on the other, the present is a most propitious moment for drawing public attention to this vast science, as being the true centre and foundation of that practical knowledge which is needed as a guide, and also as a stimulus, for practical everyday life. It will, of course, be instantly objected that the subject is indeed vast--much too vast. But not too vast, surely, if, by means of a very simple principle, we select out what is of immediate definite use, and necessary for everybody, from what may be, by the majority, safely left on one side. We shall then get, on this side, the highly specialised Biology of the laboratory with its minute researches and nicely calculated experiments, and, on that, what we may, for our present purpose, call Common-sense Biology.