of the embryo point to a worm-like
animal as the next stage. Constant swimming in the water would give the
animal a definite head, with special groups of nerve-cells, a definite
tail, and a two-sided or evenly-balanced body.

We mean that those animals would be fittest to live, and multiply most,
which developed this organisation. Sense-organs would now appear in the
head, in the form of simple depressions, lined with sensitive cells, as
they do in the embryo; and a clump of nerve-cells within would represent
the primitive brain. In the vast and varied worm-group we find
illustrations of nearly every step in this process of evolution.

The highest type of worm-like creature, the acorn-headed
worm--_Balanoglossus_--takes us an important step further. It has
gill-openings for breathing, and a cord of cartilage down its back. We
saw that the human embryo has a gill-apparatus, and that, comparing the
lancelet and the sea-squirt, the backbone must have begun as a string of
cartilage-cells. We are now on firmer ground, for there is no doubt that
all the higher land-animals come from a fish ancestor. The shark, one of
the most primitive of fishes in organisation, probably best suggests
this ancestor to us. In fact, in the embryonic development of the human
face there is a clear suggestion of the shark.

Up to this period the story of evolution had run its course in the sea.
The area of dry land was now increasing, and certain of the primitive
fishes adapted themselves to living on land. They walked on their fins,
and used their floating-bladders--large air-bladders in the fish, for
rising in the water--to breathe air. We not only have fishes of this
type in Australia to-day, but we have the fossil remains of similar
fishes in the Old Red Sandstone rocks. From mud-fish the amphibian would
naturally develop, as it did in the coal-forest period. Walking on the
fins would strengthen the main stem, the broad paddle would become
useless, and we should get in time the bony five-toed limb. We have many
of these giant salamander forms in the rocks.

The reptile now evolved from the amphibian, and a vast reptile
population spread over the earth. From one of these early reptiles the
birds were evolved. Geology furnishes the missing link between the bird
and the reptile in the _Archaeopteryx_, a bird with teeth, claws on its
wings, and a reptilian tail. From another primitive reptile the
important group of the mammals was evolved. We