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. 
