ZOOLOGY, the science which is concerned with the study of animals. Its scope embraces all conceivable modes of study, not only of individual animals, but of entire faunas and of the relation of animals with one another, with plants and with the non-living environment.
The number of animals, both living and extinct, which has been investigated is very great, perhaps approaching two million separate forms. It is, therefore, necessary to provide every dif ferent kind of animal with a name by which it can be recognized by zoologists in all countries, and to establish a classification by which the name of any animal may be determined.
The more obvious and easily discovered characters which dis tinguish animals from one another are those of their external appearance ; body shape, nature of appendages, sense organs, character of the skin and its derivatives, colour and size, enable the ordinary man to identify the animals with which he is brought into contact, either as enemies, or as the objects of his sports and the source of his food supplies, or from the interest of their attractive appearance and habits.
The earliest attempts to name and classify animals were based exclusively on these characters, and the smaller divisions of a classification are still necessarily founded on them.
But it soon became obvious that, from external appearance alone, it was impossible to provide a key to the whole animal kingdom, and zoologists were driven, to a study of the internal structure of animals.
The necessary beginning of such studies is an investigation of the anatomy of individual animals. The French anatomists of the i8th century published a series of works, each dealing with the anatomy of a single specified individual, and disclaimed any belief that the facts that they recorded would necessarily be true for any other individual, even though it was apparently of the same kind.
Anatomical investigations of this character were carried out at first entirely by dissection of the animal's carcass with knives, scissors and other implements, a technique which, in skilful hands, can yield an astonishing amount of accurate information, but which is necessarily incapable of revealing many structures, either because they are too small to be seen or otherwise undiscoverable. The progress of anatomy has depended very largely on advances in methods of investigation.
The most important of all such is that which came from the introduction of the microscope. The possibility of magnifying an animal at once greatly extended the scope of zoology, because it enabled the structure of very small animals to be investigated and, indeed, revealed an immense variety of forms, previously unknown because they were too small to be seen with the naked eye. At the same time microscopical examination showed that the organs of which an animal was composed were themselves complex, each consisting of a variety of tissues themselves con structed of smaller units, the cells. Work along all these lines is still continuing; each year sees the publication, though in decreas ing numbers, of accounts of the structure of additional animals, and corrections of former accounts of anatomical detail, so that zoologists have now reliable accounts of the structure of an immense number of animals from all regions of the animal kingdom.
These accounts are, in part, based on dissection, but in most cases depend on microscopical investigations made by many methods. The fact that the cells which compose animals, and the various substances of which they are formed, have different chemi cal and electrical affinities for dyes, makes it possible to differenti ate between them by staining animals. or parts of animals, before subjecting them to microscopical examination. The application of this method was made possible by the discovery of processes by which an animal may be cut into a series of slices, sections often only one-hundredth of a millimetre thick. From a study of such a series of sections it is possible to reconstruct the whole anatomy of a microscopic animal in very great detail. But in many cases the object of such investigations is not to discover the anatomy of a whole animal, but to describe the fine structure of some one organ, such as an eye or ear, elucidating the arrangement of the cells of which it is composed. For such studies the term histology (q.v.) has been introduced. They only acquire significance when they are linked up with concurrent physiological investigations, so that an attempt can be made to relate the observed structures with the function that they subserve during the animal's life.