to fix the precise limits of the types which connect the two great Kingdoms together, and to determine where vegetable life ends and animal life begins. In respect of form, internal structure, power of motion, they closely resemble each other as in the case of the DIATOMACEAE and DESMIDIAE, two of the lower order of miscroscopic plants, and the sea-anemones, sea-mats, sponges, corals, etc. Like the Vegetable Kingdom, animals are limited to certain areas by the conditions of climate and soil, the environment of an animal determining its development and survival. There are various systems of classification of animals. Linnaeus divided them into six great classes: Mammalia, Birds, Fishes, Amphibious Animals, Insects and Worms. Cuviers' more scientific arrangement comprised the four sub-Kingdoms, Vertebrata, Mollusca, Articulata, and Radiata. The classifications of more recent times have been largely determined by the consideration of the forms of animals as influenced by evolution, and the facts of heredity and adaptation to their environment. The late Professor Huxley divded the Animal Kingdom into the sub-Kingdoms, Vertebrata, Annuloda, Annuloida, Coelenterata, Infusoria, and Protozoa.
This class of animals comprises all the ordinary quadrupeds, and includes all those vertebrate animals in which some part or other of the skin, during some portion of life, is provided with hair, and whose young are nourished for a longer or shorter period by means of a special fluid-milk, secreted by special glands. The mammalia have warm blood, i.e., blood the normal temperature of which is usually retained in any atmosphere. Among the mammalia are classed the whales, dolphins and porpoises, whose bodies are particularly adapted for aquatic life, and, who like the rest of the class, nourish their young by their milk. The heart of mammals is similar to that of birds, and has four distinct chambers, two auricles and two ventricles, and respiration is carried on by means of two lungs situated in the chest cavity.
The following are the general characteristics of the Mammalia. As noticed above, the bodies of nearly the whole class are covered with hair—a kind of clothing which is both soft and warm, little liable to injury, and bestowed in proportion to the necessities of the animal, the climate of the country it inhabits, and the nature of its environment. In all the higher orders of animals, the head is the principal seat of the organs of sense. In it are placed the eyes, the ears, nose, and the mouth. Through the last they receive nourishment. The mouth contains the teeth, which, in most of the mammalia, are used not only for the mastication of food, but also as weapons of defence. They are inserted into two movable jaws, and incisors are so placed that their sharp edges may easily be brought in contact with the food, in order that its fibres may