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. [Illustration: Fig. XIV.] 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