ricious fact, but an unavoidable phenomenon. It
has its place in that vast, orderly concourse which has successively
risen in the past, has introduced the present, and is preparing the way
for a predestined future. From point to point in this vast progression
there has been a gradual, a definite, a continuous unfolding, a
resistless order of evolution. But in the midst of these mighty changes
stand forth immutable the laws that are dominating over all.

If we examine the introduction of any type of life in the animal series,
we find that it is in accordance with transformation, not with creation.
Its beginning is under an imperfect form in the midst of other forms,
of which the time is nearly complete, and which are passing into
extinction. By degrees, one species after another in succession more and
more perfect arises, until, after many ages, a culmination is reached.
From that there is, in like manner, a long, a gradual decline.

Thus, though the mammal type of life is the characteristic of the
Tertiary and post-Tertiary periods, it does not suddenly make its
appearance without premonition in those periods. Far back, in the
Secondary, we find it under imperfect forms, struggling, as it were, to
make good a foothold. At length it gains a predominance under higher and
better models.

So, too, of reptiles, the characteristic type of life of the Secondary
period. As we see in a dissolving view, out of the fading outlines of
a scene that is passing away, the dim form of a new one emerging, which
gradually gains strength, reaches its culmination, and then melts
away in some other that is displacing it, so reptile-life doubtfully,
appears, reaches its culmination, and gradually declines. In all this
there is nothing abrupt; the changes shade into each other by insensible
degrees.

How could it be otherwise? The hot-blooded animals could not exist in
an atmosphere so laden with carbonic acid as was that of the primitive
times. But the removal of that noxious ingredient from the air by the
leaves of plants under the influence of sunlight, the enveloping of its
carbon in the earth under the form of coal, the disengagement of its
oxygen, permitted their life. As the atmosphere was thus modified,
the sea was involved in the change; it surrendered a large part of its
carbonic acid, and the limestone hitherto held in solution by it was
deposited in the solid form. For every equivalent of carbon buried in
the earth, there was a