The metatarsal bones never exceed five in number, and the phalanges follow the same numerical rule as in the manus, never exceeding three in each digit. Moreover, the first digit, counting from the tibial side, or hallux, resembles the pollex of the hand in always having one segment less than the other digits. As the function of the hind foot is more restricted than that of the hand the modifications of its structure are less striking. In the Cetacea and the Sirenia it is entirely wanting, though in some existing members of the first-named order rudiments of the bones of both the first and second segments of the limb have been detected, and a femur is present in the Miocene Sirenian _Halitherium_. IV. THE DIGESTIVE SYSTEM. _General Considerations._—The search after the purpose which every modification of structure subserves in the economy is always full of interest, and, if conducted with due caution and sufficient knowledge of all the attendant circumstances, may lead to important generalisations. It must always be borne in mind, however, that adaptation to its special function is not the only cause of the particular form or structure of an organ, but that this form, having in all probability been arrived at by the successive and gradual modification of some other different form from which it is now to a greater or less degree removed, has other factors besides use to be taken into account. In no case is this principle so well seen as in that of the organs of digestion. These may be considered as machines which have to operate upon alimentary substances in very different conditions of mechanical and chemical combination, and to reduce them in every case to the same or precisely similar materials; and we might well imagine that the apparatus required to produce flesh and blood out of coarse fibrous vegetable substances would be different from that which had to produce exactly the same results out of ready-made flesh or blood; and in a very broad sense we find that this is so. Thus, if we take a large number of carnivorous animals, belonging to different fundamental types, and a large number of herbivorous animals, and strike a kind of average of each, we shall find that there is, pervading the first group, a general style, if we may use the expression, of the alimentary organs, different from that of the others. That is to say, there is a specially carnivorous and a specially herbivorous modification of these parts. But, if function were the only element which has guided such modification, it might be inferred that, as one form must be supposed to be best adapted in its relation to a particular kind of diet, that form would be found in all the animals consuming such diet. But this is far from being the case. Thus the Horse and the Ox, for instance—two animals whose food in the natural state is precisely similar—are most different as regards the structure of their alimentary canal, and the processes involved in the preparation of that food.