ceeds, remains are found in great abundance,
and we see that there must have been a vast and varied population of the
Amphibia on the shores of the Carboniferous lagoons and swamps. There
were at least twenty genera of them living in what is now the island of
Britain, and was then part of the British-Scandinavian continent. Some
of them were short and stumpy creatures, a few inches long, with weak
limbs and short tails, and broad, crescent-shaped heads, their
bodies clothed in the fine scaly armour of their fish-ancestor (the
Branchiosaurs). Some (the Aistopods) were long, snake-like creatures,
with shrunken limbs and bodies drawn out until, in some cases, the
backbone had 150 vertebrae. They seem to have taken to the thickets, in
the growing competition, as the serpents did later, and lost the use of
their limbs, which would be merely an encumbrance in winding among
the roots and branches. Some (the Microsaurs) were agile little
salamander-like organisms, with strong, bony frames and relatively long
and useful legs; they look as if they may even have climbed the trees in
pursuit of snails and insects. A fourth and more formidable sub-order,
the Labyrinthodonts--which take their name from the labyrinthine folds
of the enamel in their strong teeth--were commonly several feet in
length. Some of them attained a length of seven or eight feet, and had
plates of bone over their heads and bellies, while the jaws in their
enormous heads were loaded with their strong, labyrinthine teeth. Life
on land was becoming as eventful and stimulating as life in the waters.

The general characteristic of these early Amphibia is that they very
clearly retain the marks of their fish ancestry. All of them have tails;
all of them have either scales or (like many of the fishes) plates of
bone protecting the body. In some of the younger specimens the gills can
still be clearly traced, but no doubt they were mainly lung-animals. We
have seen how the fish obtained its lungs, and need add only that this
change in the method of obtaining oxygen for the blood involved certain
further changes of a very important nature. Following the fossil
record, we do not observe the changes which are taking place in the
soft internal organs, but we must not lose sight of them. The heart, for
instance, which began as a simple muscular expansion or distension of
one of the blood-vessels of some primitive worm, then doubled and
became a two-chambered pump in the fish,