.
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. The development of the frog is a
reminiscence of it, on the lines of the embryonic law which we saw
earlier. An animal, in its individual development, more or less
reproduces the past phases of its ancestry. So the free-swimming
jelly-fish begins life as a fixed polyp; a kind of star-fish (Comatula)
opens its career as a stalked sea-lily; the gorgeous dragon-fly is at
first an uncouth aquatic animal, and the ethereal butterfly a worm-like
creature. But the most singular and instructive of all these embryonic
reminiscences of the past is found in the fact that all the higher
land-animals of to-day clearly reproduce a fish-stage in their embryonic
development.

In the third and fourth weeks of development the human embryo shows four
(closed) slits under the head, with corresponding arches. The bird, the
dog, the horse--all the higher land animals, in a word, pass through the
same phase. The suggestion has been made that these structures do not
recall the gill-slits and gill-arches of the fish, but are folds due
to the packing of the embryo in the womb. In point of fact, they appear
just at the time when the human embryo is only a fifth of an inch
long, and there is no such compression. But all doubt as to their
interpretation is dispelled when we remove the skin and examine the
heart and blood-vessels. The heart is up in