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, 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.
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