I. The naturalist who meets with an object for the first time, instinctively asks the question:—What is this object? This question leads to another:—With what other objects shall I class it? To what group, and, in the first place, to what kingdom does it belong? Is it a mineral, a plant, or an animal? The answer is not always easy. We know that, in what may be called the basis of each kingdom, there are ambiguous forms, whose nature has long been, and still is, the subject of contention among naturalists. We know that polyps were long regarded as plants, and that nullipores, at first taken for polyps, are now divided between the vegetable and mineral kingdoms; and, finally, we know that even now, botanists and zoologists dispute over certain diatoms and transfer them from one kingdom to the other. Similarly the question has been asked:—What is man? and it has been answered from several points of view. To the naturalist it has but one meaning, and signifies, in which kingdom must man be placed? or better: is man an animal? In spite of all the differences which a comparison of man with the mammalia presents, should he be classed with them? This question is similar to that which Peysonnel is said to have asked himself, when, struck by the special phenomena presented by the coral, he asked himself whether the object before him was a vegetable. It is evident that, in order to solve the first problem which arises from a study of the natural history of man, we must have a clear idea what are these great groups of beings, which are called kingdoms; we must give an account of the characters which distinguish and separate them from each other, and then of their true scientific meaning. It will be sufficient for the purpose to explain the well-known laws of Linnæus, supplementing the theory of the immortal Swede by some ideas borrowed from Pallas and de Candolle, and by one of the fundamental conceptions which Adamson and A. L. de Jussieu have almost equally contributed to introduce into science. II. It is impossible for anyone, whether learned or otherwise, not to recognise at once the difference between two kinds of objects very distinct from each other: inanimate bodies and organised beings. These are the two groups into which Pallas has divided kingdoms under the name of empires. Their distinction is generally easy, and I shall confine myself to recalling some of the most essential differences.