now develops a partition in
the auricle (upper chamber), so that the aerated blood is to some extent
separated from the venous blood. This approach toward the warm-blooded
type begins in the "mud-fish," and is connected with the development
of the lungs. Corresponding changes take place in the arteries, and we
shall find that this change in structure is of very great importance in
the evolution of the higher types of land-life. The heart of the higher
land-animals, we may add, passes through these stages in its embryonic
development.

Externally the chief change in the Amphibian is the appearance of
definite legs. The broad paddle of the fin is now useless, and its main
stem is converted into a jointed, bony limb, with a five-toed foot,
spreading into a paddle, at the end. But the legs are still feeble,
sprawling supports, letting the heavy body down almost to the ground.
The Amphibian is an imperfect, but necessary, stage in evolution. It is
an improvement on the Dipneust fish, which now begins to dwindle very
considerably in the geological record, but it is itself doomed to give
way speedily before one of its more advanced descendants, the Reptile.
Probably the giant salamander of modern Japan affords the best
suggestion of the large and primitive salamanders of the Coal-forest,
while the Caecilia--snake-like Amphibia with scaly skins, which live
underground in South America--may not impossibly be degenerate survivors
of the curious Aistopods.

Our modern tailless Amphibia, frogs and toads, appear much later in the
story of the earth, but they are not without interest here on account of
the remarkable capacity which they show to adapt themselves to different
surroundings. There are frogs, like the tree-frog of Martinique, and
others in regions where water is scarce, which never pass through the
tadpole stage; or, to be quite accurate, they lose the gills and tail in
the egg, as higher land-animals do. On the other hand, there is a modern
Amphibian, the axolotl of Mexico, which retains the gills throughout
life, and never lives on land. Dr. Gadow has shown that the lake in
which it lives is so rich in food that it has little inducement to leave
it for the land. Transferred to a different environment, it may pass to
the land, and lose its gills. These adaptations help us to understand
the rich variety of Amphibian forms that appeared in the changing
conditions of the Carboniferous world.

When we think of the diet o