HOW THE MAMMALS DEVELOPED 215 



stance, in the cow. Since, however, we have come 

 to disbelieve in the transmission of acquired charac- 

 ters, this explanation will no longer serve. We must 

 content ourselves with saying that, by whatever acci- 

 dent the nipple arose, the success of it when present 

 determined its selection by nature and its consequent 

 persistence. With increase in its function has come 

 increase in the size of the glands. Lower animals 

 which, like the hog, produce a large number of off- 

 spring, possess a large number also of these glands. 

 With the diminishing number of young and greater 

 care of them as we rise in the scale has come also a 

 diminishing number of breasts in the female. Whether 

 those on the front of the body should persist, or those 

 on the rear, depends upon other factors in the life 

 of the animal. Hoofed animals, perhaps because their 

 best weapon is the hoof and they can there best pro- 

 tect their young, have retained them in the rear of 

 the body. In the group of animals known as the 

 primates, including monkeys, apes, and man, the habit 

 of holding the young in the arms for protection has 

 determined the persistence of the breasts upon the 

 chest rather than the abdomen. 



It is interesting to notice that the habit of the ele- 

 phant of protecting its young by means of its tusks 

 has also resulted in a similar position of the milk 

 glands. 



