the immediate principles of food, and animal life destroyed them; the various excretions of animal life were the natural ferment of vegetal life, and the latter purified the air, contaminated by animal emanations; finally, that function of the organism which is most continuous, namely, respiration, consisted, in animals, in the absorption of oxygen, followed by exhalation of carbonic acid, while in plants it consisted in the absorption of carbonic acid, followed by exhalation of oxygen. In this way the respiration of plants would decompose the carbonic acid produced by the respiration of animals, thus preserving the normal constitution of the atmosphere.
The famous experiments of Claude Bernard on the glycogenic function of the liver, revealing, as they did, the formation in the liver of animals of one of the most important of these immediate principles, to wit, sugar, delivered on this apparently philosophic and well-established theory a severe blow, from which it could not recover. Soon, a very different theory, one no less philosophic in its general form, was proposed; and this theory was so bound up with the tendencies of modern science and philosophy, that its success was assured from the outset. In place of the harmonic contrast of the two kingdoms, we have now the functional unity of living Nature. Our readers have not forgotten the lectures delivered during several years by Claude Bernard, at the Paris Museum of Natural History, in which he has developed this grand conception. Instead of comparing together animals and plants, pointing out their differences, as the usual course has been, Bernard enumerates their resemblances; and this simple change in the point of view at once gives to the ensemble of the facts a very different meaning.
But, still there appear to remain some fundamental differences, chiefly with regard to respiration.
Since the date of the early researches, which made out the respiration of plants to be an exhalation of oxygen resulting from the decomposition of the carbonic acid of the atmosphere, sundry not very recent experiments have to a certain extent limited the bearings of the original conclusions. It was soon discovered that this mode of respiration is subordinated to the action of solar light, and that it occurs only in the leaves and in the green portions of the plant, the coloration being due to the presence of a special principle called chlorophyll; and thus the chlorophyll came to be regarded as the organ, the essential agent of plant-respiration. Next, the discovery was made that the flowers of a color different from green, and even the green portions themselves when placed in the dark, not only do not absorb carbonic acid out of the atmosphere, and then exhale the oxygen of that acid; they go further, and do the very opposite, absorbing oxygen and giving up carbonic acid, just as animals do.
Hence the assignment to plants of a second mode of respiration, known as nocturnal, as opposed to the other mode, called the diurnal
