100 REPRODUCTION 



time are used and oxygen taken through them. Similarly 

 the digestive tract has been developed merely for the use 

 it was going to serve, not for any value it had before birth. 

 One can readily see that birth is not in any sense the 

 beginning of life. It is merely a change from one kind of 

 life on the part of the embryo to another kind. It is 

 similar to that which the toad makes when it passes from 

 living and breathing in water to living in the air. 



6. Care after Birth. It must not be thought that the 

 mammal young are, or ought to be, completely independent 

 at birth. Just as in birds, there is a period in which the 

 young demand a great deal of care from the parent. This 

 is accomplished in many ways, but the particular form of 

 care in which we are interested is that peculiar to mammals 

 and which gives them their name. This is by means of 

 the secretion known as milk. It is produced by skin 

 glands which have become highly modified and often 

 greatly enlarged. It consists of water, of salts from the 

 blood, of a form of sugar, of considerable oil which in fine 

 particles gives the milk its whiteness, and of certain 

 proteins. It is thus a complete food for the young animal. 

 From the point of view of successful reproduction, milk 

 makes it possible for more of the young to live than could 

 possibly come to maturity without it. Carrying the young 

 in the body and furnishing them with a complete food 

 when first born are two most wonderful inventions, so to 

 speak. They go far toward insuring success to the group 

 of animals that use them. 



All that has been said about the reproduction and 

 development of mammals applies to man. 



