But the necessary complication of structure does not stop even here; for the communication of one function of a superior order imposes the necessity of communicating still another. Locomotion cannot be exercised without perception; sensation is indispensable to volition, and volition, of course, to voluntary motion. A being endowed with the power of moving from place to place, without possessing the power of perceiving external objects, must be speedily destroyed. The communication of sensation to a creature fixed immovably to a single spot, conscious of the approach of bodies, but incapable of avoiding their contact, would be not only useless but pernicious, since it would be to make a costly provision for the production of pain, and nothing else; but the communication of locomotion without sensation would be as unwisely defective, as the former would be perniciously expensive; since it would be to endow a being with a faculty, the exercise of which would be fatal to it for the want of a second faculty to guide the first. Nor could the possession of locomotion, without the further possession of sensation, be otherwise than fatal, for another reason. Consciousness is not necessary to nutrition as performed by the plant, but it is indispensable to nutrition as performed by the animal: for if the food of the animal be not always on the same spot with itself; if it be under the necessity of searching for it, and of conveying it, when found, into the interior of its body, it must, of course, possess the power of perceiving it when within its reach, and of apprehending and appropriating it by an act of volition, of none of which actions is it capable without the possession of sensation. Again, then, we see that, in order to secure harmonious action, function must be put in relation with function. In order to prevent jarring and mutually-destructive action, function must be superadded to function, and throughout the animal creation the complication of structure, which is necessary for the accomplishment of these ends, is given without parsimony, but without profusion: nothing is given which is not needed, nothing is withheld which is required.