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, the crocodiles
had assumed the modern type of structure. Notwithstanding this, the
crocodiles of the chalk are not identically the same as those which lived
in the times called "older tertiary," which succeeded the cretaceous
epoch; and the crocodiles of the older tertiaries are not identical with
those of the newer tertiaries, nor are these identical with existing
forms. I leave open the question whether particular species may have
lived on from epoch to epoch. But each epoch has had its peculiar