ZOOLOGY (Gr. soon, an amimal, and logos, a discourse), the science which has for its subject the animal kingdom (q.v). This science, itself a branch of natural history (q.v.), is divided into a number of branches, which are often pursued as distinct sciences, the subject being too large to be thoroughly studied except in this manner; although it is 'lso necessary that the results of investigation in particular departments should be brought together, so that the animal kingdom may be viewed as a whole, and the rela tions of the most widely different groups of animals to each other determined. The number of species of animals is far greater than that of plants, and the diversity among them is also greater, so that a division of the science of zoology into branches relating to different groups very naturally takes place. Thus, the branch of zoology which has the mammalia for its subject has been called masology (Gr. mason, a teat), an unhappily chosen and essentially incorrect term, which has not come into general use; and it is a curious circumstance that this, which may be call the highest branch of zoology, has no popularly received name. It is otherwise with the branches of zoology relating to the inferior classes of vertebrate; that which has birds for its subject is universally known as ornithology (q.v.); that which reldtes to reptiles is herpetology (q.v.), and the subordinate branch relating to serpents is sometimes called ophiology; that which relates to fishes is Ichthyology (q.v.). Among invertebrate animals the great group of mollusca is the subject of the science of malacology (q.v.), although this term is not in such general use as some of those already mentioned; and when shells rather than the animals which bear them are considered, the term concholon (q.v.) is employed. No particular term is commonly applied to the branches of zoology which treat of the crustacea, arachnida, etc. ; but that which relates to insects is universally known as entomology (q.v.), and the term helmin Otology (q.v.) is employed to designate that which has worms for its subject. No similar terms are used for the branches of this science which relate to other groups of invertebrate.
The science of zoology, however, divides itself into distinct sciences, not only in accordance with the divisions of the animal kingdom, but also with regard to particular branches of the subject which may be studied either in relation to animals generally or to any particular species. Thus, anatomy (q.v.) may be regarded as a branch of zoology,
when the term zoology is taken in its largest sense, as including man along with the inferior animals, and ethnology (q.v,) must in like manner be considered as belonging to it. The anatomy of the inferior animals is sometimes called zootomy, and the term com parative anatomy is employed when their structure is studied in relation to that of man, and the structure of one division of the animal kingdom in relation to that of the others. Physiology (q.v.) is one of the most important branches of zoology ; and with it that branch of chemistry which treats of animal substances is closely connected. A very interesting branch of zoology is that which relates to the habits and instincts of animals. It can hardly be said to have been constituted into a separate science, but has received much attention from those naturalists who have devoted themselves to the study of par ticular groups of animals.
We have no evidence that the study of zoology was prosecuted to any considerable extent before the time of Aristotle. In his hands it became at once a science, and the foundations of a system of classification were laid. No artificial system of classification, has ever been proposed in zoology, like the sexual system of Liunams in botany; but from the very first to the present day a natural grouping of animals has always been attempted. To this the widely marked distinctions between the principal 'groups almost unavoid ably led. Aristotle brought to bear upon the subject the highest powers both of observa tion and of generalization, and some of the groups established by him still retain their place• in the most modern systems. Aristotle divided the whole animal kingdom into two great sections, the highest, enaima, consisting of animals having blood (i.e., red blood), and the lower, anaima, of animals having a colorless fluid instead of blood, the former corresponding to the vertebrata and the latter to the invertebrata of modern zoologists.