The Dog, belongs to the division of animals termed VERTEBRATED, (see 'The Horse', 2d edition, page 106), because it has a cranium or skull, and a spine or range of VERTEBRAE proceeding from it. It ranks under the 'class' MAMMALIA, because it has teats, by which the female suckles her young; the 'tribe' UNGUICULATA, because its extremities are armed with nails; the 'order' DIGITIGRADES, because it walks principally on its toes. The 'genus' CANIS has two tubercular teeth behind the large carnivorous tooth in upper jaw; and the 'sub-genus familiaris', the DOG, has the pupils of the eye circular, while those of the wolf are oblique, and those of the fox upright and long. There has been some dispute whether the various species of dogs are of different origin, or sprung from one common source. When we consider the change that climate and breeding effect in the same species of dog, and contrast the rough Irish or Highland greyhound with the smoother one of the southern parts of Britain, or the more delicate one of Greece, or the diminutive but beautifully formed one of Italy, or the hairless one of Africa or Brazil--or the small Blenheim spaniel with the magnificent Newfoundland; if also we observe many of them varied by accident, and that accidental variety diligently cultivated into a new species, altogether different in form or use, we shall find no difficulty in believing that they might be derived from one common origin. One of the most striking proofs of the influence of climate on the form and character of this animal, occurs in the bull-dog. When transported to India he becomes, in a few years, greatly altered in form, loses all his former courage and ferocity, and becomes a perfect coward. It is probable that all dogs sprang from one common source, but climate, food, and cross-breeding caused variations of form, which suggested particular uses; and these being either designedly or accidentally perpetuated, the various breeds of dogs thus arose, and they have become numerous in proportion to the progress of civilization. Among the ruder, or savage tribes, they possess but one form; but the ingenuity of man has devised many inventions to increase his comforts: he has varied and multiplied the characters and kinds of domestic animals for the same purpose, and hence the various breeds of horses, and cattle, and dogs.