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 crocodiles; though all, since the chalk, have
belonged to the modern type, and differ simply in their proportions, and
in such structural particulars as are discernible only to trained eyes.

How is the existence of this long succession of different species of
crocodiles to be accounted for?

Only two suppositions seem to be open to us--Either each species of
crocodile has been specially created, or it has arisen out of some
pre-existing form by the operation of natural causes.

Choose your hypothesis; I have chosen mine. I can find no warranty for
believing in the distinct creation of a score of successive species of
crocodiles in the course of countless ages of time. Science gives no
countenance to such a wild fancy; nor can even the perverse ingenuity
of a commentator pretend to discover this sense, in the simple words
in which the writer of Genesis records the proceedings of the fifth and
sixth days of the Creation.

On the other hand, I see no good reason for doubting the necessary
alternative, that all these varied species have been evolved from
pre-existing crocodilian forms, by the operation of causes as completely
a part of the common order of nature, as those which have effected the
changes of the inorganic world.

Few will venture to affirm that the reasoning which applies to
crocodiles loses its force among other animals, or among plants. If one
series of species has come into existence by the operation of natural
causes, it seems folly to deny that all may have arisen in the same way.

A small beginning has led us to a great ending. If I were to put the bit
of chalk with which we started into the hot but obscure flame of burning
hydrogen, it would presently shine like the sun. It seems to me that
this physical metamorphosis is no false image of what has been
the result of our subjecting it to a jet of fervent, though nowise
brilliant, thought to-night. It has become luminous, and its clear rays,
penetrati