§ 3. We have become quite familiar with the idea of an evolution of structures throughout the ascending types of animals. To a considerable degree we have become familiar with the thought that an evolution of functions has gone on _pari passu_ with the evolution of structures. Now, advancing a step, we have to frame a conception of the evolution of conduct, as correlated with this evolution of structures and functions. These three subjects are to be definitely distinguished. Obviously the facts comparative morphology sets forth, form a whole which, though it cannot be treated in general or in detail without taking into account facts belonging to comparative physiology, is essentially independent. No less clear is it that we may devote our attention exclusively to that progressive differentiation of functions, and combination of functions, which accompanies the development of structures--may say no more about the characters and connections of organs than is implied in describing their separate and joint actions. And the subject of conduct lies outside the subject of functions, if not as far as this lies outside the subject of structures, still, far enough to make it substantially separate. For those functions which are already variously compounded to achieve what we regard as single bodily acts, are endlessly recompounded to achieve that co-ordination of bodily acts which is known as conduct. We are concerned with functions in the true sense, while we think of them as processes carried on within the body; and, without exceeding the limits of physiology, we may treat of their adjusted combinations, so long as these are regarded as parts of the vital _consensus_. If we observe how the lungs aërate the blood which the heart sends to them; how heart and lungs together supply aërated blood to the stomach, and so enable it to do its work; how these co-operate with sundry secreting and excreting glands to further digestion and to remove waste matter; and how all of them join to keep the brain in a fit condition for carrying on those actions which indirectly conduce to maintenance of the life at large; we are dealing with functions. Even when considering how parts that act directly on the environment--legs, arms, wings--perform their duties, we are still concerned with functions in that aspect of them constituting physiology, so long as we restrict our attention to internal processes, and to internal combinations of them.