There is another set of organs in the mammalian skin. At the base of each hair lies an oil gland. The function of these is to pour out a substance which spreads along each hair and over the surface of the body. The outside of the skin is always dead, and would easily crack were it not for the constant secretion of this oil. In winter, when the blood circulates less freely and these glands consequently pour out less oil, the supply frequently runs short. If what little is poured out is too frequently removed by washing, the skin becomes brittle, and, on bending a joint, the epidermis cracks. The gloss of the hair is due to the oil thus poured out. This oil becomes one ingredient in the milk produced by the transformed gland. But there is another important constituent. When one does unaccustomed manual work the ordinary result is the formation of a blister. The epidermis, or scarfskin, becomes detached from the dermis, or true skin, and the space between the two rapidly fills with the fluid portion of the blood, known as lymph. The fact that no blood vessels have been broken in this detachment results in there being no red corpuscles in this fluid. Wherever a cavity forms in the body lymph is liable to enter it. The milk glands of the mammals are modified oil glands. The fluid which they now pour out is no longer exactly the old oil with the addition of the lymph. Undoubtedly in the past the first milk was more like this simple mixture. There seems no doubt that the breasts of to-day are the enlarged and modified oil glands of earlier mammals. In one of the most primitive of our mammals the young simply lick certain bare spots on the surface of the mother's abdomen. As higher forms arise there develops a smaller or larger mound with a distinct projection, about which the lips of the offspring can easily fasten. Lamarck would have said that the suction of the infant had produced such a mound, and that this had been transmitted to later offspring until it had grown to be the highly developed organ we now find, for instance, in the cow. Since, however, we have come to disbelieve in the transmission of acquired characters, this explanation will no longer serve. We must content ourselves with saying that, by whatever accident the nipple arose, the success of it when present determined its selection by nature and its consequent persistence. With increase in its function has come increase in the size of the glands. Lower animals which, like the hog, produce a large number of offspring, possess a large number also of these glands. With the diminishing number of young and greater care of them as we rise in the scale has come also a diminishing number of breasts in the female. Whether those on the front of the body should persist, or those on the rear, depends upon other factors in the life of the animal. Hoofed animals, perhaps because their best weapon is the hoof and they can there best protect their young, have retained them in the rear of the body.