istance; the density of the sea must
have increased, its volume must have become less; the constitution of
the atmosphere must have varied, especially in the amount of water-vapor
and carbonic acid that it contained; the barometric pressure must have
declined.

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 cap