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- 
