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 



