The fact that likeness of structure, which compels naturalists to group certain animals into a family in spite of possible unlikeness in size or form, is accompanied by resemblance in quality, is well illustrated by the family Bovidæ (Latin boves, "cattle") which includes goats, sheep, antelopes, and oxen; for all of these in flesh, products and disposition, are alike suited to the requirements of men, and especially of mankind in a social civilization. This family of animals furnishes us with nearly all of our milk, butter, and cheese; with flesh food, woolen clothing, leather goods, horn, gelatin, etc.: and gives us such servants as the ox and goat; while sportsmen find in it the most fascinating of their larger game.

The distinctive feature of this most useful of animal tribes is the possession of hollow horns, properly so called. Horn is a chitinous material developed from the skin, and not dissimilar to hair; indeed it would be no great stretch of facts to say that a cow's horn was composed of agglutinated hairs. These horns are sheaths that grow over cores of bone—outgrowths of the skull—increase in size until their wearers are mature, grow at the base as fast as worn at the tips, and are never shed. They may be borne by the males alone, or by both sexes; or the males may have horns far larger than those of the female, as in the sheep; and in a few cases both sexes are hornless. No family is more difficult to subdivide, for the various forms intergrade inextricably.