animal function of animals, and constitutes their fourth distinctive character.
Thus wrote Cuvier in 1828. But in the fourth and fifth decades of this century, the greatest and most rapid revolution which biological science has ever undergone was effected by the application of the modern microscope to the investigation of organic structure; by the introduction of exact and easily manageable methods of conducting the chemical analysis of organic compounds; and finally, by the employment of instruments of precision for the physical measurement of the forces which are at work in the living economy.
That the semi-fluid contents (which we now term protoplasm) of the cells of certain plants, such as the Charæ, are in constant and regular motion, was made out by Bonaventura Corti a century ago but the fact, important as it was, fell into oblivion, and had to be rediscovered by Treviranus in 1807. Robert Brown noted the more complex motions of the protoplasm in the cells of Tradescuntia in 1831; and now such movements of the living substance of plants are well-known to be some of the most widely prevalent phenomena of vegetable life.
Agardh, and other of the botanists of Cuvier's generation, who occupied themselves with the lower plants, had observed that, under particular circumstances, the contents of the cells of certain water-weeds were set free and moved about with considerable velocity, and with all the appearances of spontaneity, as locomotive bodies, which, from their similarity to animals of simple organization, were called "zoospores."
Even as late as 1845, however, a botanist of Schleiden's eminence deals very sceptically with these statements; and his scepticism was the more justified, since Ehrenberg, in his elaborate and comprehensive work on the infusoria, had declared the greater number of what are now recognized as locomotive plants to be animals.
At the present day, innumerable plants and free plant-cells are known to pass the whole or part of their lives in an actively locomotive condition, in no wise distinguishable from that of one of the simpler animals; and, while in this condition, their movements are, to all appearance, as spontaneous — as much the product of volition — as those of such animals.
Hence the teleological argument for Cuvier's first diagnostic character — the presence in animals of an alimentary cavity, or internal pocket, in which they can carry about their nutriment, has broken down so far, at least, as his mode of stating it goes. And with the advance of microscopic anatomy the universality of the fact itself among animals has ceased to be predicable. Many animals of even complex structure, which live parasitically within others, are wholly devoid of an alimentary cavity. Their food is provided for them, not only ready cooked but ready digested, and the alimentary canal, become superfluous, has disappeared. Again, the males of most rotifers have no digestive apparatus; as a German naturalist has remarked, they devote themselves entirely to the Minnedienst, and are to be reckoned among the few realizations of the Byronic ideal of a lover. Finally, amidst the lowest forms of animal life, the speck of gelatinous protoplasm, which constitutes the whole body, has no permanent digestive cavity or mouth, but takes in its food anywhere; and digests, so to speak, all over its body.
But although Cuvier's leading diagnosis of the animal from the plant will not stand a strict test, it remains one of the most constant of the distinctive characters of animals. And if we substitute for the possession of an alimentary cavity, the power of taking solid nutriment into the body and there digesting it, the definition so changed will cover all animals, except certain parasites, and the few and exceptional cases of non-parasitic animals which do not feed at all. On the other hand, the definition thus amended will exclude all ordinary vegetable organisms.
Cuvier himself practically gives up his second distinctive mark when he admits that it is wanting in the simpler animals.
The third distinction is based on a completely erroneous conception of the chemical differences and resemblances between the constituents of animal and vegetable organisms, for which Cuvier is not re-
