These changes, and very many more that might be mentioned, must have taken place not in a discontinuous but in an orderly manner, since the master-fact, the decline of heat, that was causing them, was itself following a mathematical law.

But not alone did lifeless Nature submit to these inevitable mutations; living Nature was also simultaneously affected.

An organic form of any kind, vegetable or animal, will remain unchanged only so long as the environment in which it is placed remains unchanged.  Should an alteration in the environment occur, the organism will either be modified or destroyed.

Destruction is more likely to happen as the change in the environment is more sudden; modification or transformation is more possible as that change is more gradual.

Since it is demonstrably certain that lifeless Nature has in the lapse of ages undergone vast modifications; since the crust of the earth, and the sea, and the atmosphere, are no longer such as they once were; since the distribution of the land and the ocean and all manner of physical conditions have varied; since there have been such grand changes in the environment of living things on the surface of our planet—­it necessarily follows that organic Nature must have passed through destructions and transformations in correspondence thereto.

That such extinctions, such modifications, have taken place, how copious, how convincing, is the evidence!

Here, again, we must observe that, since the disturbing agency was itself following a mathematical law, these its results must be considered as following that law too.

Such considerations, then, plainly force upon us the conclusion that the organic progress of the world has been guided by the operation of immutable law—­not determined by discontinuous, disconnected, arbitrary interventions of God.  They incline us to view favorably the idea of transmutations of one form into another, rather than that of sudden creations.

Creation implies an abrupt appearance, transformation a gradual change.

In this manner is presented to our contemplation the great theory of Evolution.  Every organic being has a place in a chain of events.  It is not an isolated, a capricious 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.