down of protein compounds; but, as the formation of new protein within the body of the plant usually exceeds this consumption, plants, as manufacturers of protein, are broadly distinguished from animals. It is now known, however, that many very highly-organized flowering plants are carnivorous, and digest and make use of the protein of the animals which they capture, and it is probable that the potato-fungus and many other parasitic plants obtain all their protein ready made, like animals; and, as it is impossible to show that none of the lower animals have the power to make protein for themselves, this distinction cannot be made the basis of an absolute line between the two groups.
The difference in the process of respiration in animals and plants is well known. Animals while in a state of vital activity absorb oxygen from the air; and this is given off from their bodies, usually united with carbon, as carbonic acid. The green plants, on the contrary, absorb carbonic acid, which is separated by the chlorophyll, under the influence of sunlight, into carbon, which is appropriated by the plant, and oxygen, which is given off and may be again taken up by an animal. This difference is made use of in the arrangement of an aquarium; enough green plants being placed in the water to absorb the excess of carbonic acid given off by the animals, and to supply the oxygen for their respiration. The difference is not by any means absolute, however, since the vital changes of the plant are dependent, like those of the animal, upon oxidation, and result in the formation of carbonic acid. The colorless plants, like animals, absorb oxygen and give off carbonic acid. This is also true of green plants which are not exposed to light; but in the latter plants this process is normally masked and hidden by the opposite process already spoken of.
It is plain, from what has been said, that the separation of organisms into two great group—animals and plants—is convenient and natural, and that the distinctions between them are real but not absolute; and it is possible to define, that is give, all the characteristics which are distinctive of an animal, without implying or assuming that all animals conform with the definition to the same degree, or that no plant shares any of the characteristics. Since the lower representatives of the two groups resemble each other more closely than the higher forms, and since all positive characteristics gradually disappear as we approach the point of union or origin, we must, in order to give our definition any definiteness whatever, neglect the lowest and simplest forms, and consider only the more specialized.
As shown by the highest forms, an animal may be described morphologically as an organism made up of cells, which are usually without a cell-wall or membrane. In the adult, the individuality of these cells is usually lost, since they are united to form membranes, tissues, and fibres. In nearly all animals the tissues thus built up from cells fall into four groups—epithelium, connective tissue, muscular tissue, and nervous tissue.