or less from one another, which are bound together and integrated into a single organism. Living cells and organisms are not static structures which are fixed and stable in character, but they are systems which are undergoing continual change. They are like the river, or the whirlpool, or the flame, which are never at two consecutive moments composed of the same particles but which nevertheless maintain a constant general appearance; in short they are complex systems in dynamical equilibrium. The principal physiological processes by which all living things maintain this equilibrium are:
2. Metabolism, or the transformation of matter and energy within the living thing, in the course of which some substances are oxidized into waste products, with the liberation of energy, while other substances are built up into protoplasm, each part of the cell converting food substances into its own particular substance by the process of assimilation.
3. Reproduction, or the capacity of organisms to give rise to new organisms, of cells to give rise to other cells, and of parts of cells to give rise to similar parts by the process of division.
4 Irritability, or the capacity of receiving and responding to im-
