the throat, as in the fish,
and has only two chambers, as in the fish (not four, as in the bird
and mammal); and the arteries rise in five pairs of arches over the
swellings in the throat, as they do in the lower fish, but do not in
the bird and mammal. The arrangement is purely temporary--lasting only
a couple of weeks in the human embryo--and purposeless. Half these
arteries will disappear again. They quite plainly exist to supply fine
blood-vessels for breathing at the gill-clefts, and are never used, for
the embryo does not breathe, except through the mother. They are a most
instructive reminder of the Devonian fish which quitted its element and
became the ancestor of all the birds and mammals of a later age.

Several other features of man's embryonic development--the budding of
the hind limbs high up, instead of at the base of, the vertebral column,
the development of the ears, the nose, the jaws, etc.--have the same
lesson, but the one detailed illustration will suffice. The millions of
years of stimulating change and struggle which we have summarised have
resulted in the production of a fish which walks on four limbs (as the
South American mud-fish does to-day), and breathes the atmosphere.

We have been quite unable to follow the vast changes which have meantime
taken place in its organisation. The eyes, which were mere pits in
the skin, lined with pigment cells, in the early worm, now have a
crystalline lens to concentrate the light and define objects on the
nerve. The ears, which were at first similar sensitive pits in the skin,
on which lay a little stone whose movements gave the animal some sense
of direction, are now closed vesicles in the skull, and begin to be
sensitive to waves of sound. The nose, which was at first two blind,
sensitive pits in the skin of the head, now consists of two nostrils
opening into the mouth, with an olfactory nerve spreading richly
over the passages. The brain, which was a mere clump of nerve-cells
connecting the rough sense-impressions, is now a large and intricate
structure, and already exhibits a little of that important region (the
cerebrum) in which the varied images of the outside world are combined.
The heart, which was formerly was a mere swelling of a part of one of
the blood-vessels, now has two chambers.

We cannot pursue these detailed improvements of the mechanism, as we
might, through the ascending types of animals. Enough if we see more or
less clearly how the