These chemical principles are founded upon facts—upon observation. As I have said, carnivorous animals are never fat, because they feed upon a substance rich in nitrogen—flesh; which flesh makes flesh, and very little fat. They have no belly, because flesh, taken in small quantity, suffices for one day, or twenty-four hours.

It has been objected that the carnivora do not always obtain food when hungry, and that they are often obliged to chase their prey for a long time before catching it. This is true; but on the other hand, carnivorous animals, when domesticated and fed upon meat, are not more fat, and have no belly. The celebrated traveller, Levaillant, in his Travels in Africa, says that he has seen, in the southern part of the continent, flocks of gazelles, which live in the interior, numbering from ten to fifty thousand. These flocks are almost continually on the move; they travel from north to south, and from south to north. Those of the flock which are in advance, and in the enjoyment of a rich pasturage, frequently come upon the borders of the settlements of Cape Colony, and are fat; those composing the centre of the herd are less fat; while those in the rear are extremely poor, and dying with hunger. Being thus stayed in their course by the presence of man, they retrace their steps; but those which composed the rear are now in advance, and regain their fat, while those which were in advance become the rear, and lose fat. Notwithstanding the vast numbers which daily perish, their natural increase suffices to maintain the integrity of the herd. In connexion with my subject I may state that these flocks are always accompanied or followed by lions, leopards, panthers and hyenas, which kill as many of them as they please for food, devour a part, and leave the rest to the jackals and other small carnivorous animals, which follow upon their steps. Now, these lions, panthers, leopards and hyenas, which need make but the slightest exertion to find food when hungry, are never fat.

It has been said, by way of objection to my system, that butchers are generally fat, due to their living upon meat. Now, I have made some enquiries in this matter, and have satisfied myself that butchers, as a general thing, are not fond of meat, but live chiefly upon vegetable food, and usually drink a great deal. It has been said also that their good condition is due to the atmosphere (filled with animal miasm) in which they live, a supposition which has yet to be proven. Again, it has been said that hogs can be fattened upon horse-flesh. My reply is, that they drink at the same time a large amount of water. And here I may remark, that the lard of hogs thus fattened upon flesh is soft and watery, and is considered by dealers to be of little value. It is evidently not due to the flesh upon which these hogs are fed, that their fat is soft and watery, but to the great amount of fluid they imbibe.

On the other hand, those animals which are enormously fat, live exclusively upon vegetables, and drink largely. The hippopotamus, for example, so uncouth in form from its immense amount of fat, feeds wholly upon vegetable matter—rice, millet, sugar-cane, &c. Naturalists long entertained the opinion that this animal, living mostly in the water, fed chiefly upon fish. It is now, however, well ascertained that the hippopotamus never touches fish, and is wholly a vegetable feeder.

The walrus, which, according to Buffon, seems to afford the connecting link between amphibious quadrupeds and the cetacea, is a veritable mass of fat, and lives exclusively upon marine herbage. The walrus of Kamschatka measures from twenty to twenty-three feet in length, sixteen to eighteen feet in circumference, and weighs from six to eight thousand pounds.

The following fact may be cited as a remarkable proof that the quantity of fat in any animal is mainly dependent on the character of its food: Among the whale tribe, those monsters in size, that of Greenland (Balæna mysticetus of Linnæus) possesses the greatest amount of blubber, and it feeds upon zoophytes, of which many resemble as much in character the plant as the animal. The fin-backed whale (Balæna böops of Linnæus), which does not feed upon mucilaginous matter, but upon small fish, has a much thinner layer of blubber than the former. The sperm whale or cachalot (Balæna physalus of Linnæus), which feeds on mackerel, herrings, and northern salmon, although nearly as long as the Greenland whale, is much thinner. The layer of blubber is not so thick as in the fin-backed, and yields only ten or twelve tuns of oil; while the Greenland whale yields fifty, sixty, and even eighty tuns.

Now, chemistry, as we have said, furnishes a rational explanation of these facts. With the exception of flesh, all alimentary substances (the mucilaginous, the gummy, the saccharine, the aqueous, &c.) consist of carbon and hydrogen, and fat is composed of the same elements. Success in the treatment of disease would be more frequent, if medical practitioners would pay greater attention to the chemistry of the vital functions; and the reason why certain articles of diet have a greater tendency than others to the formation of fat, would, by the aid of the exact science of chemistry, be rendered self-evident.