find what seem to be the
transitional types in the rocks of South Africa. The scales gave way to
tufts of hair, the heart evolved a fourth chamber, and thus supplied
purer blood (warm blood), the brain profited by the richer food, and the
mother began to suckle the young. We have still a primitive mammal of
this type in the duck-mole, or duck-billed platypus (_Ornithorhyncus_)
of Australia. There are grounds for thinking that the next stage was an
opossum-like animal, and this led on to the lowest ape-like being, the
lemur. Judging from the fossil remains, the black lemur of Madagascar
best suggests this ancestor.

The apes of the Old and New Worlds now diverged from this level, and
some branch of the former gave rise to the man-like apes and man. In
bodily structure and embryonic development the large apes come very
close to man, and two recent discoveries have put their
blood-relationship beyond question. One is that experiments in the
transfusion of blood show that the blood of the man-like ape and man
have the same action on the blood of lower animals. The other is that we
have discovered, in Java, several bones of a being which stands just
midway between the highest living ape and lowest living race of men.
This ape-man (_Pithecanthropus_) represents the last of our animal and
first of our human ancestors.

_IV.--Evolution of Separate Organs_

So far, we have seen how the human body as a whole develops through a
long series of extinct ancestors. We may now take the various systems of
organs one by one, and, if we are careful to consult embryology as well
as zoology, we can trace the manner of their development. It is, in
accordance with our biogenetic law, the same in the embryo, as a rule,
as in the story of past evolution.

We take first the nervous system. In the lowest animals, as in the early
stages of the embryo, there are no nerve-cells. In the embryo the
nerve-cells develop from the outer, or skin layer, of cells. This,
though strange as regards the human nervous system, is a correct
preservation of the primitive seat of the nerves. It was the surface of
the animal that needed to be sensitive in the primitive organism. Later,
when definite connecting nerves were formed, only special points in the
surface, protected by coverings which did not interfere with the
sensitiveness, needed to be exposed, and the nerves transmitted the
impressions to the central brain.

This development is found in the anima