That nutrition and locomotion may go on together, it is clear, then, that there must be provided a distinct apparatus for containing food, a distinct apparatus for deglutition, a distinct apparatus for circulation, a distinct apparatus for respiration, a distinct apparatus for excretion, and so on; and that, in this manner, the communication of a single function of a superior order renders a modification not merely of one but of many inferior functions absolutely indispensable, in order to adjust the one to the other, and to enable them to act in harmony.
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.
4. As we ascend in the scale of organization, numerous functions being carried on, and numerous organs constructed for performing them, it is obvious that the range of each function must be proportionally extended; the range necessarily increasing with the multiplication of organ and function: and this is another cause of the unavoidable complication of structure. Slight consideration will suffice to show the necessary connexion between an extended range of action and complication of structure. Take, as an example, the organic function of respiration: respiration is the function by which air is brought into contact with food; it is the completion of digestion. The sole end of all the apparatus that belongs to this function is to bring the air and the food into a certain degree of proximity. Now, when all the substances that enter into the composition of the body of an animal are slight, delicate, and permeable to air (as in fig. XIV.), and when the body is always surrounded by air, air must at all times be in contact with the particular organ that contains the food, no less than with the general system to which the food is distributed. In this case, to construct a separate apparatus for containing air would be useless, because wherever food is, there air must be, since it constantly permeates every part of the body.
When, on the other hand, the tissues are so firm and dense as to be impermeable; when they are folded into bulky and complex organs, and when these organs are placed in situations to which the external air cannot reach, the construction of a separate apparatus for respiration is indispensable. The respiratory apparatus consists either of organs for carrying air to the food, or of organs for carrying food to the air. The one or the other is adopted, according to the nature of the body. If the size of the animal be small; if the tissues which form the solid portion of its body be delicate in texture; if, at the same time, the wants of its economy require that its food should be highly aërated (for there is the closest connexion between energy of function and perfect aëration of the food), an apparatus of sufficient magnitude to aërate the food in a high degree would occupy the entire bulk of the body. In such a case, it is easier to carry air to the food than food to the air; it is better to make the entire body a respiratory organ, than to construct a respiratory organ disproportioned to the magnitude of the body. Air-tubes diffused through every part of the body, and opening on its external surface, would obviously afford to every point of the system an easy access of air. By an expedient of this kind the system might be highly aërated, while the respiratory apparatus would occupy but a comparatively small space; the function might be performed on an extended scale, while there would be no necessity for encumbering a minute body with a bulky organ. And this is the mode in which respiration is carried on in large tribes of creatures, whose body is small in size and delicate in texture, and the functions of whose economy are performed with energy (fig. XV.).
The Achilles Butterfly of South America (Papilio Achilles),
showing the tracheæ on the upper and under side of the
wings.
But this contrivance will not do when the animal is of large magnitude; when its body is divided into numerous compartments; when these compartments extend far beneath the external surface; when important organs are placed in deeply-seated cavities; and when the substances that compose the organs are dense, hard, thick, and convoluted. To construct air-tubes of the requisite diameter and length, always open, always in a condition to permit the ingress and egress of an adequate current of air to and from the remotest nook and corner of a body such as this, would be difficult, if not impossible. At all events, it is easier, in such a case, to carry the food to the air, than the air to the food. But, for the accomplishment of this purpose, what is necessary? An organ for containing food; an organ for containing air; vessels to carry food to and from the receptacle of the aliment; vessels to carry air to and from the receptacle of the air; expedients to expose a stream of food to a current of air; and, finally, tubes to carry out to the system the product of this complicated operation. Accordingly, a reservoir of food and a reservoir of air; an apparatus by which both are conveyed to their respective receptacles; and an apparatus by which both are brought into contact sufficiently close to admit of their mutual action, are all combined in the lung of the animal, and in the mechanism by which its movements are effected. The object is accomplished, but the apparatus by which it is effected is as complex in structure as it is efficient in action; the result simple; the means by which the result is secured, highly complicated.
And if this be true of an inferior or organic function, it is still more strikingly true of a superior or animal function. The relation is still stricter between the complexity of the apparatus of sensation and the range of feeling, than between the complexity of the apparatus of respiration and the range of the respiratory process. The greater the number of the senses, the greater the number of the organs of sense; the more accurate and varied the impressions conveyed by each, the more complex the structure of the instrument by which they are communicated; the more extended the range of the intellectual operations, the larger the bulk of the brain, the greater the number of its distinct parts, and the more exquisite their organization. From the point of the animal scale, at which the brain first becomes distinctly visible, up to man, the basis of the organ is the same; but, as the range of its function extends, part after part is superadded, and the structure of each part becomes progressively more and more complex. The evidence of this, afforded by comparative anatomy, is irresistible, and the interest connected with the study of it can scarcely be exceeded.