Thus we get very clear suggestions of the transition from water
to land. We must, of course, conceive it as a slow and gradual
adaptation. At first there may have been a rough contrivance for
deriving oxygen directly and partially from the atmosphere, as
the water of the lake became impure. So important an advantage
would be fostered, and, as the inland sea became smaller, or its
population larger or fiercer, the fishes with a sufficiently
developed air-breathing apparatus passed to the land, where, as
yet, they would find no serious enemy. The fact is beyond
dispute; the theory of how it occurred is plausible enough; the
consequences were momentous. Great changes were preparing on the
land, and in a comparatively short time we shall find its new
inhabitant subjected to a fierce test of circumstances that will
carry it to an enormously higher level than life had yet reached.
I have said that the fact of this transition to the land is
beyond dispute. The evidence is very varied, but need not all be
enlarged upon here. The widespread Dipneust fishes of the
Devonian rocks bear strong witness to it, and the appearance of
the Amphibian immediately afterwards makes it certain.
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