ANIMAL KINGDOM. All natural objects are referred by naturalists to three great divisions, called the Kingdoms of Nature. These are respectively called after the objects they include, the Mineral Kingdom, Vegetable Kingdom, and Animal Kingdom. Although at first sight nothing would appear easier than defining these great groups, in such a manner as to afford an easy means of distinction, it is nevertheless one of considerable difficulty. The difficulty, however, does not lie in the typical object of each kingdom, as a rock, an oak-tree, and a man, but in applying the definition to those objects which lie a.s it were on the limits of each kingdom. The line is perhaps better drawn between minerals, and plants and animals, than between the latter two. It is usual to speak of minerals as forming the Inorganic Kingdom or portion of Nature, whilst plants and animals constitute the Organic Kingdom. The great distinctive character of the Organic Kingdom, is the fact of their parts originating in and being formed out of cells which give to them many characters by which they are distinguished from minerals. Thus they are unsymmetrical, whilst minerals are myrunictrical ; they grow irregularly, whilst minerals increase in definite cryrtalline forms. Each portion of e mineral, however email, consists of the saute elements, whilst any part of a plant or animal may be differently composed to another part. The line in fact is nowhere difficult to be drawn, where the presence or absence of cells can be determined.
The distinctions between animals and Planta present greater difficulties, and perhaps no mere structural or formal difference can be found. Looked at. from one point of and animals form a great organic unity, connected together by their common modal of cellular growth and functions ; and when thus regarded, there seems to be no necessity for drawing an absolute line of distinction between one and the other. Naturalists have however regarded them as distinct, and the study of the two classes of objects have constituted the sciences of Botany and 7-xdogy. Rude definitions of various kinds have been laid down to guide the systematist in his classification of the objects belonging to each. Aristotle was one of the first who sought n distinction, and in stating that an animal possessed a mouth whilst a plant had no such organ, he gave perhaps the simplest and moat generally applicable definition that exist& lint Aristotle had not the microscope to direct his inquiries, and by the aid of this instrument beings can be made apparent to which other distinctions must be applied before they can bo arranged in one kingdom or another. Professor Kolliker describes en animalcule, the Aetinophrys sol of Ehrenberg (' Nierostoopical Journal,' Nos. i. and ii.), in which, though no mouth is found, the function of digestion is carried on by an indentation of its skin, temporarily formed for that purpose. Limuena, with no better success, gave the following definition : " Minerals grow, plants grow and live, animals grow, live, and feel." To apply this definition, we must define life and feeling, and this cannot be done in such a way as to effect the object of the naturalist. Cuvier thought
the possession of a stomach a sufficient distinction for the animal kingdom, but the nature of a stomach must first be understood, and here we have no absolute structural character to guide us. It was at one time a favourite distinction that animals have the power of motion, and that plants are fixed, but we know now that many plants move, whilst many animals are fixed. One of the most recent and philoso phical of physiological writers says " A plant is an organised being, whose vital powers are directed solely to the performance of formative operations, by which its fabric is not merely built up in the first instance, but is continually receiving additions during the term of its existence; and any movements which it may exhibit are destined solely for the furtherance of these operations, and must be regarded as originating in physical or vital forces. Ou the other hand an animal is an organised being, whose vital powers are not merely directed to the construction and maintenance of its corporeal fabric, but are also subservient to the operations of the conscious mind, which involve a continual disintegration of the structures that minister to them ; on the repair of which, rather than on the extension of the fabric, after it bas attained its full development, the formative energy is chiefly expended ; and of the movements which it may exhibit, though a part are still to be regarded as directly dependent (like those of plants) on causes inherent in its material organisation, there is another part, small though it may often be, in which the consciousness and spontaneity of the individual are necessarily concerned, and which must therefore be distinguished as originating in psychical causes." (Carpenter, ' Principles of Physiology.') In this way the naturalist and physiologist have tried to contend with the difficulty. Within the last few yearn chemistry has invaded the domain of the anatomist, and supplied him with materials for determining the problem of the difference of minimal and vegetable life. The substances found in 'animal', and plants are found in a great measure to be formed of four elements, carbon, hydrogen, oxygen, and nitrogen. At one time nitrogen was supposed to distinguish animal from vegetable substances. It is now known, not only that plants contain nitrogen, but that they supply this and the three other elements to the animal system. It is found that these four elements are always present in the protoplasm 11 Udell*, cytoblast, or primordial utriele, from which the cells of all plants and animals are find formed. So that they are universally necessary in pinata and animals, cud have hence been called Organic Elements. Three compounds of three of these elements, carbon, hydrogen, and oxygen, and called cellulose, sugar, and starch, were at one time thought to be peculiar to plants. It is new known that cellulose is present in many anbnals, and that auger is very generally present in certain animal textures and fluids. Starch yet remains to be found In the animal kingdom, and its presence in doubtful structures In still regarded as evidence of their vegetable nature.