The above statement as to the organic nature of the food suggests some interesting considerations.
In the first place, it seems to shew that the living animal of to-day pre-supposes another organization of yesterday ; — that its iadividual descent from two creatures of the same species is accompanied by a less evident, but quite as real, transmission of substance from several previous beings. In short, that the greater part of its entire mass might be regarded as the sum of various legacies, whieh have been bequeathed to the existing organism by the various plants and animals that lived before it.
In the next place, it indicates a fixed and definite relation between the plant and the animal. The former is thus the chief agent in the constructive chemistry of the latter:— a necessary link in that chain of processes which builds up organic principles, out of the elements of inorganic nature, or out of those simple products into which the particles of the animal body are finally converted by its waste during life, or its putrefaction after death. The carbonic acid given off by the living or dead animal may especially exemplify the latter remark ; converted as it is, by the vegetable, from a poisonous gas into a class of substances which are in the highest sense alimentary, and essential to the life of the animal.
And lastly, since animal and vegetable life are thus complementary to each other, alike in their broader features and their minuter details, we may conjecture that, in the present disposition of our planet, they form what is in fact a tolerably constant magni tude : —a sum of organized life, the amount of which is subject to but very slight varia tion from one time to another. Nay more, we may almost suspect that the total of animal existence — the composition of which ranges thus regularly through vegetable or ganization as an essential part of its cycle of metamorphosis —is in the main equally constant and fixed. Created by what even modern science must be content to own as a miracle, in the strictest sense of the word, it seems not improbable that animal, as well as vegetable life, is su.stained in consonance with some vast law of this kind. According to such a law, each by each, and both together, would make up certain constant units ; the innumerable constituent fractions of which might vary within vast limits without ex ercising any effect on their respective sums.
And thus the world of life around us would but parallel that perpetual flux, but un altered quantity, which the chemist has long predicated of the various materials which compose the inorganic globe we inhabit.
But if, on the one hand, the animal is in capable of constructing its complex tissues front the simple elements of inorganic nature, still, on the other hand, it is not bound down by such rigorous chemical necessities, as to demand a food possessing an exact identity of composition with itself. A large propor tion of the animal creation feed on a vegetable diet, the constituents of which deviate con siderably front those of their own mass. And but very few of even the more carnivorous animals are in the habit of devouring their own species. Finally, though the blood forms the pabulum of all the tissues, and hence closely approaches their total composition, still it does not appear to form even an advantageous article of food, far less an indispensable one.
And while such considerations may suf fice to show, that there is no true identity be tween the food and the tissues in general, the progress of modern physiological chemistry plainly indicates, that an identity of this kind would be equally impossible in detail. Thus it is not improbable, that the tissues of every individual possess chemical peculiarities more or less specific to himself. And it is all but certain, that the various proximate principles isolated by the chemist are not definite com binations of certain elements in equivalent proportions — as are the salts, acids, and alka lies of the inorganic world —but rather ever varying mixtures. Those various forms of protein which it is so convenient to distinguish by the names of albumen, fibrin, and casein, may indeed be separated from the tissues of animals, and even of vegetables, by the same rough processes ; and may therefore respec tively exhibit the closest resemblance in their composition and properties. But an accu rate analysis would probably show, that the organic substance represented by either of these terms is never precisely identical in any two specimens. It is the total of a number of constituents, the result of a variety of pro cesses, the end of a serial metamorphosis : rather than a definite and specific compound of carbon, oxygen, hydrogen, and nitrogen.