Thus far we have spoken of the change in the method by which the young are brought to such a stage of development that they can stand the outer air. One of the improved differences between the mammals and other animals lies in the method by which they nourish their young for some time after birth. The very word mammals signifies an animal who is in the true sense of the word a mamma. This name for mother is given to her because of the fact that she possesses what are technically known as mammary glands, or, in simpler language, breasts. It would seem as if here we had an entirely new organ. No other animal gives nourishment to its young in such fashion; all mammals do. What is the origin of the habit? How did the organ arise? A part of an animal's body that has the power to gather material from the blood and pour it out in the shape of fluid is known as a gland. Sometimes a whole organ does nothing else. Sometimes small glands are scattered through, or over, the surface of another organ. There are two kinds of glands in the skin of the mammal. The best known and most frequently thought of are those which pour out the perspiration. These have a double function. In the first place they assist in keeping the temperature of the body uniform. When we are too warm they pour out a watery fluid over the surface of the body. If the air is dry enough and our body not too closely protected by clothing, this perspiration passes off in the form of vapor. All evaporation requires heat, which in this case is extracted from the body. So soon as the temperature returns to its normal level the flow of perspiration ceases. The other function of the sweat glands is to take from the blood some of the waste matters of the body and pour them out upon the surface. This is done in order that the body may free itself from substances which, if they were to accumulate, would have a poisonous effect upon its action. It is this function of the sweat glands which makes it necessary for us to bathe the surface of our bodies with water. Dirt, in the ordinary sense of the word, is not harmful to a sound skin. Our reason for bathing is really to remove the wastes which we ourselves have poured upon the surface of the skin. These, if allowed to remain, soon decompose, like all nitrogenous substances, and become very offensive. They may then be reabsorbed into the skin and nature's effort to throw them off has been in vain. These glands, since they contain waste matter, could not possibly yield food for the young. They would poison and not nourish. Hence, whatever the breasts may be, they are not altered sweat glands.