414 PHILOSOPHY OF ZOOLOGY. 
Woman’s milk contains more cream than the milk of the 
cow; and the cheese and butter are so intimately united 
that it is very difficult to effect their separation. 'The 
milk of all the other animals which has been examined, 
contains similar ingredients, but in variable proportions. 
The most nutritious portion of the milk is usually con- 
sidered as the cheese, which approaches in its nature to al- 
bumen. The claims, however, of all the other ingredients 
to be considered as nutritious, are equally strong. They 
are constantly present, and consist of ingredients needed 
by the young system. 
During the first periods of infancy, milk is the only diet 
administered to the young animal. All the parts of the 
mouth are at the same time soft, and adapted exclusively for 
sucking, without injuring the nipple. By the time the teeth 
make their appearance, the young animal is beginning to 
imitate the actions of its mother, and attempting to eat of 
the food which she employs. Weaning now takes place, 
and independent existence may be said to commence. Be- 
tween this period and old age, there are several important 
stages which may be briefly noticed. The casting of the 
teeth is the mark of increased activity in the system. The 
old ones drop out, or are worn away, and new ones, better 
adapted for acting upon the food now made use of, supply 
their place. Puberty speedily takes place, and the de- 
velopment and influence of the organs of the reproductive 
system, communicate to the individual impressive marks of 
sexual distinction. Maturity succeeds, with its accompa- 
nying strength and energy. Old age at length approaches, 
with its feebleness and inactivity, and a diminished power 
of generating heat or cold. The hair approaches to white- 
ness. The teeth fall out, and the means of obtaining food 
thus failing, the body sinks to rest. Man is the only ani- 
mal that can counteract the fatal consequences attending 
