For instance, some of the lower animals, as the fresh-water POLYP, present nearly a uniform structure throughout their whole substance; and every part of them consequently performs the same function. There is not one organ for digestion, another for circulation, a third for respiration, and so on; but all these functions are performed by the same structure, and are performed, therefore, in a rude and imperfect manner. Any portion of the creature possesses all the requisites for its own nutrition, and is, so, independent of the remainder, and can live alone. Hence, the polyp may be divided into a number of pieces, each of which goes on living. Gradually, as we ascend from these lowly beings to the higher classes of animals, we find organs and functions more and more distinct from another; a Stomach is provided for the work of digestion, a Heart for circulation, Lungs for respiration. Each of these organs is essential to the existence of the others and of every part of the body; and they are all maintained in harmonious co-operation by the presiding influence of the nervous system. Or, trace one of the _functions_ in illustration of the same principle. Take the function of Locomotion, which has an especial relation to our present subject. In the LEECH and the WORM the whole length of the body is occupied in the work, one part as much as another; and still, it is but a crawl. In the FISH the whole body is buoyed up by the water; it is flattened from side to side, and is all, from the head to the tail, concerned in the lateral stroke by which the animal is driven along; the side fins, which are the representatives of limbs, doing little beyond serving to guide and balance. In the other VERTEBRATES the work of locomotion is so far concentrated as to be assigned, almost entirely, to the limbs. All _four_ limbs are in most of them devoted to it; while the bones and muscles of the trunk are only indirectly concerned in it. In MAN, however, _two_ limbs only are assigned to this important office. In him, therefore, the concentration of locomotive power, in other words the principle of division of labour, is carried out to the greatest extent--a disposition which affords one of the many proofs that the construction of his body combines with the faculties of his mind to place him at the head of the animal kingdom.