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 realisations 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 responsible, as it was current among contemporary chemists.  It is now established that nitrogen is as essential a constituent of vegetable as of animal living matter; and that the latter is, chemically speaking, just as complicated as the former.  Starchy substances, cellulose and sugar, once supposed to be exclusively confined to plants, are now known to be regular and normal products of animals.  Amylaceous and saccharine substances are largely manufactured, even by the highest animals; cellulose is widespread as a constituent of the skeletons of the lower animals; and it is probable that amyloid substances are universally present in the animal organism, though not in the precise form of starch.