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.