Science. Science is classified truth. Men study the heavenly bodies, note their characteristics, observe their movements, and define their relationships; and having verified their deductions by repeated experiments, arrange the truths they have discovered into systems, and by classifying their knowledge reduce it to a science: this science they call Astronomy. Astronomy is thus the classified arrangement of all known truths concerning the heavenly bodies. Geology, similarly, is the classified arrangement of all known truths concerning the material structure of the Earth.

The Kingdoms of Nature. The Natural World has been variously divided for the purposes of study. LinnÆus divided it into three kingdoms; (I) the Mineral kingdom (II) the Vegetable kingdom and (III) the Animal kingdom, thus naming the three kingdoms in the order of their natural geneses. The Mineral kingdom comprises the inorganic forms of nature,—those which have no organism and which can only increase by external addition. The Vegetable and Animal kingdoms comprise the organic life of nature,—those forms which are provided with means for promoting their own development and propagating species. The Vegetable kingdom, while easily distinguishable from the Mineral kingdom is in some of its forms so similar to the lower forms of animal life as to suggest relationship between the two; while the Animal kingdom, beginning with the lower forms which approximate so closely to vegetable forms, embraces the whole range of animal life and reaches its highest order in man. The science which treats of organic life as a whole is called Biology, while its two departments are separately known as Botany and Zoology. Natural History is a general term popularly applied to the study of Zoology.

Zoology. Zoology is the science of animal life. It deals with the origin of species, and the evolution of the varied forms of animated nature, and treats of the structure, habits, and environment of all living creatures. Scientifically speaking, Zoology is the classified arrangement of all known truths concerning all animal organisms.

Classification. For convenience in study the Animal kingdom is divided into seven Sub-kingdoms, each of which is further divided into classes. These Sub-kingdoms are known as: I Vertebrata, II Arthropoda, III Mollusca, IV Echinodermata, V Vermes, VI Coelenterata, and VII Protozoa. Sub-kingdom I, Vertebrata, includes all animals distinguished by the possession of VertebrÆ or back-bones, and its classes are I Mammalia:—animals that suckle their young; II Aves:—Birds; III Reptilia:—Reptiles; IV Batrachia:—Frogs, Toads, etc.; and V Pisces:—Fishes. Sub-kingdom II, Arthropoda, includes the Insect families, etc., which it also divides into classes. Sub-kingdom III, Mollusca, animals of the cuttle-fish order, including limpets, oysters, and slugs. Sub-kingdom IV, Echinodermata, a large number of marine animals, such as the star-fish and the sea-urchin. Sub-kingdom V, Vermes, the various classes of worms. Sub-kingdom VI, Coelenterata, corals and sponges, etc., etc., and Sub-kingdom VII, Protozoa, protoplasms and the lowest forms of animal life. This volume is devoted to the illustration of the first of these sub-kingdoms, the Vertebrata, with its five classes, Mammalia, Aves, Reptilia, Batrachia and Pisces.