ermis, or true skin, and the
space between the two rapidly fills with the fluid portion of the
blood, known as lymph. The fact that no blood vessels have been broken
in this detachment results in there being no red corpuscles in this
fluid. Wherever a cavity forms in the body lymph is liable to enter
it.

The milk glands of the mammals are modified oil glands. The fluid
which they now pour out is no longer exactly the old oil with the
addition of the lymph. Undoubtedly in the past the first milk was more
like this simple mixture. There seems no doubt that the breasts of
to-day are the enlarged and modified oil glands of earlier mammals. In
one of the most primitive of our mammals the young simply lick certain
bare spots on the surface of the mother's abdomen. As higher forms
arise there develops a smaller or larger mound with a distinct
projection, about which the lips of the offspring can easily fasten.
Lamarck would have said that the suction of the infant had produced
such a mound, and that this had been transmitted to later offspring
until it had grown to be the highly developed organ we now find, for
instance, in the cow. Since, however, we have come to disbelieve in
the transmission of acquired characters, this explanation will no
longer serve. We must content ourselves with saying that, by whatever
accident 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 offspring,
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 protect
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 elephant of
protecting its young by means of its tusks has also resulted in a
similar position of the milk glan