HOW THE MAMMALS DEVELOPED 217 
a firm protective shell and hidden away from sight 
by being buried in the ground. In the mammals com- 
paratively few eggs are produced at one time. These 
are fertilized within the body of the parent, are at- 
tached to the parent, and absorb her blood. No shell 
is needed because nothing will kill the developing off- 
spring that is not likely to injure the parent. Not 
only do the young feed upon the blood of the mother 
up to the time of birth, but they are practically de- 
pendent upon this same blood after birth. Though 
they do not take it directly from the veins, the milk 
is, none the less, the transformed blood of the mother. 
This assures the young of food as well as of protec- 
tion. Best of all, the young are provided with the 
companionship of the mother. Now for the first time 
animals learn by example. Heretofore they have been 
born with a nearly undeviating instinct; now intelli- 
gence begins to arise. They can imitate their mother. 
Heretofore no acquired characters affected the young. 
In the mammals, although the young cannot inherit 
the acquired habits of the parents, they can get them 
by imitation, which serves nearly as well. 
There is, however, a more wonderful advantage 
that comes from the close attachment between mother 
and offspring. This intimate relationship brings 
about an affection of the mother for her young hereto- 
fore unknown in the animal world. It is somewhat 
