We should
see mammals, birds, reptiles, fishes, insects, snails, and the like,
clearly recognisable as such, and yet not one of them would be just the
same as those with which we are familiar, and many would be extremely
different.
From that time to the present, the population of the world has undergone
slow and gradual, but incessant changes. There has been no grand
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.
And 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.
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