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. (§3 ¶1)

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 connexions 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 re-compounded to achieve that co-ordination of bodily acts which is known as conduct. (§3 ¶2)

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. But we enter on the subject of conduct when we begin to study such combinations among the actions of sensory and motor organs as are externally manifested. Suppose that instead of observing those contractions of muscles by which the optic axes are converged and the foci of the eyes adjusted (which is a portion of physiology), and that instead of observing the co-operation of other nerves, muscles, and bones, by which a hand is moved to a particular place and the fingers closed (which is also a portion of physiology), we observe a weapon being seized by a hand under guidance of the eyes. We now pass from the thought of combined internal functions to the thought of combined external motions. Doubtless if we could trace the cerebral processes which accompany these, we should find an inner physiological co-ordination corresponding with the outer co-ordination of actions. But this admission is consistent with the assertion, that when we ignore the internal combination and attend only to the external combination, we pass from a portion of physiology to a portion of conduct. For though it may be objected that the external combination instanced, is too simple to be rightly included under the name conduct, yet a moment’s thought shows that it is joined with what we call conduct by insensible gradations. Suppose the weapon seized is used to ward off a blow. Suppose a counterblow is given. Suppose the aggressor runs and is chased. Suppose there comes a struggle and a handing him over to the police. Suppose there follow the many and varied acts constituting a prosecution. Obviously the initial adjustment of an act to an end, inseparable from the rest, must be included with them under the same general head; and obviously from this initial simple adjustment, having intrinsically no moral character, we pass by degrees to the most complex adjustments and to those on which moral judgments are passed. (§3 ¶3)

Hence, excluding all internal co-ordinations, our subject here is the aggregate of all external co-ordinations; and this aggregate includes not only the simplest as well as the most complex performed by human beings, but also those performed by all inferior beings considered as less or more evolved. (§3 ¶4)