This condition of being, suggested the notion of Protozoa, or first animals, in the same way that the purely cellular plants, that is, each individual, consisting of a single cell, gave the idea of Protophyta, or first plants. Mr. Kirby thus expresses himself on this subject: "The first plants, and the first animals, are scarcely more than animated molecules, and appear analogues of each other; and those above them in each kingdom represent jointed fibrils."
Admitting, then, that animals as well as plants exist in the form of simple cells, and that their multiplication proceeds apparently upon the same principle in each, it is nevertheless abundantly manifest, that the cellular form of perfect individuals is infinitely more numerous in the vegetable than in the animal kingdom.
From the mosses downwards to the fungi, the whole structure of the plants consists of an aggregation of cells, more or less in number and complicate arrangement, until, through a variety of gradations, we reach the single cell as a perfect individual.
It is rather remarkable, that the lower forms of vegetables and animals seem to derive their nutriment from matter of a similar kind; and though the office of plants is as a rule, to convert inorganic into organized matter, it appears that some of the fungi may live as animals do on organic matter when in a state of solution. This, however, is uncertain; for we do not know what are the first signs of decomposition in organized bodies, and for aught we can tell, it may be perpetually going on; so far as the disengagement of carbon from the system is concerned, this is certain; but whether the nitrogenous compounds also are subject to a resolution into their elements in the living body, is another question, and not so easy of solution. The partially decomposed elements of animal structures are, however, particularly adapted for the nutrition of the lower forms of vegetation; it is, indeed, from the decaying organic matters that the fungi derive, it may be said, their entire food.
SECTION III.
SKETCH OF THE PHYSIOLOGY AND PATHOLOGY OF PLANTS AND ANIMALS.