catastrophe--no destroyer has swept away the forms of life of one period,
and replaced them by a totally new creation: but one species has vanished
and another has taken its place; creatures of one type of structure have
diminished, those of another have increased, as time has passed on. And
thus, while the differences between the living creatures of the time
before the chalk and those of the present day appear startling, if placed
side by side, we are led from one to the other by the most gradual
progress, if we follow the course of Nature through the whole series of
those relics of her operations which she has left behind. It is by the
population of the chalk sea that the ancient and the modern inhabitants
of the world are most completely connected. The groups which are dying
out flourish, side by side, with the groups which are now the dominant
forms of life. Thus the chalk contains remains of those strange flying
and swimming reptiles, the pterodactyl, the ichthyosaurus, and the
plesiosaurus, which are found in no later deposits, but abounded in
preceding ages. The chambered shells called ammonites and belemnites,
which are so characteristic of the period preceding the cretaceous, in
like manner die with it.

But, amongst these fading remainders of a previous state of things, are
some very modern forms of life, looking like Yankee pedlars among a tribe
of Red Indians. Crocodiles of modern type appear; bony fishes, many of
them very similar to existing species, almost supplant the forms of fish
which predominate in more ancient seas; and many kinds of living shell-
fish first become known to us in the chalk. The vegetation acquires a
modern aspect. A few living animals are not even distinguishable as
species, from those which existed at that remote epoch. The _Globigerina_
of the present day, for example, is not different specifically from that
of the chalk; and the same maybe said of many other _Foraminifera_. I
think it probable that critical and unprejudiced examination will show