212 THE MEANING OF EVOLUTION 
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 fre- 
quently thought of are those which pour out the per- 
spiration. 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 
