HOW THE MAMMALS DEVELOPED 213 



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 decom- 

 pose, 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 mat- 

 ter, 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. 

 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 secre- 

 tion 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 ingre^ 



