But objects belonging to the vegetable and animal kingdoms differ markedly from inert, lifeless, mineral matter. Carbon, hydrogen, oxygen and nitrogen are the most important of the few chemical elements which enter into their composition, and these elements are combined into complex organic compounds, which always contain a large percentage of water, are very unstable, and prone to spontaneous decomposition. They are composed of heterogeneous, but related, parts, termed organs, the objects possessing them being called organized bodies. Some of the lowest forms of animals have bodies whose substance is so uniform that they exhibit no definite organs, but this exception does not affect the general value of this distinction. They are always more or less definite in shape, presenting concave and convex surfaces, and being limited by curved lines. When they increase in size, or grow, as we properly term it, it is not by the addition of particles from the outside, but by the reception of foreign matter into their interior and its consequent assimilation. Certain periodic changes, which follow a definite and discoverable order, are invariably passed through by organized bodies. These changes constitute what is known as life. All the objects, then, which fulfil these conditions are said to be alive, and they all appertain either to the vegetable or the animal kingdom. The study of living objects, no matter to which kingdom they belong, is therefore conveniently called by the general name of Biology, which means a discourse on life. And as all living objects may be referred to one or other of these kingdoms, so Biology may be divided into Botany, which treats of plants, and Zoölogy, which treats of animals. Now that we have divided all organized bodies into plants and animals, it becomes necessary to inquire into the differences which subsist between them, and which will enable us to separate the kindred sciences of Botany and Zoölogy. Nothing was thought so easy by older observers than the determination of the animal or vegetable nature of any given organism, but, in point of fact, no hard-and-fast line can be drawn, in the existing state of our knowledge, between the animal and vegetable kingdoms, and it is sometimes difficult, or even impossible, to decide with positiveness whether we are dealing with a plant or an animal. In the higher orders of the two kingdoms there is no difficulty in reaching a decision, the higher animals being readily separated from the higher plants by the possession of a nervous system, of a locomotive power which can be voluntarily exercised, and of an internal cavity adapted for the reception and digestion of solid food. No so-called nervous system or organs of sense are possessed by the higher plants, although some of them doubtlessly manifest conscious and intelligent action, nor are they capable of voluntary changes of place, nor provided with any definite internal cavity, their food being generally fluid or gaseous.