this Terebratulina has peacefully propagated its species from
generation to generation, and stands to this day, as a living testimony
to the continuity of the present with the past history of the globe.

Up to this moment I have stated, so far as I know, nothing but
well-authenticated facts, and the immediate conclusions which they force
upon the mind.

But the mind is so constituted that it does not willingly rest in facts
and immediate causes, but seeks always after a knowledge of the remoter
links in the chain of causation.

Taking the many changes of any given spot of the earth's surface, from
sea to land and from land to sea, as an established fact, we cannot
refrain from asking ourselves how these changes have occurred. And when
we have explained them--as they must be explained--by the alternate slow
movements of elevation and depression which have affected the crust of
the earth, we go still further back, and ask, Why these movements?

I am not certain that any one can give you a satisfactory answer to that
question. Assuredly I cannot. All that can be said, for certain, is,
that such movements are part of the ordinary course of nature, inasmuch
as they are going on at the present time. Direct proof may be given,
that some parts of the land of the northern hemisphere are at this
moment insensibly rising and others insensibly sinking; and there is
indirect, but perfectly satisfactory, proof, that an enormous area now
covered by the Pacific has been deepened thousands of feet, since the
present inhabitants of that sea came into existence.

Thus there is not a shadow of a reason for believing that the physical
changes of the globe, in past times have been effected by other than
natural causes.

Is there any more reason for believing that the concomitant
modifications in the forms of the living inhabitants of the globe have
been brought about in other ways?

Before attempting to answer this question, let us try to form a distinct
mental picture of what has happened, in some special case.

The crocodiles are animals which, as a group, have a very vast
antiquity. They abounded ages before the chalk was deposited; they
throng the rivers in warm climates, at the present day. There is a
difference in the form of the joints of the back-bone, and in some minor
particulars, between the crocodiles of the present epoch and those which
lived before the chalk; but in the cretaceous epoch, as I have already
mentioned,