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Renewed luxuriance of the Vegetation
The insects would be one of the chief classes to benefit by the
renewed luxuriance of the vegetation. The Hymenopters
(butterflies) have not yet appeared. They will, naturally, come
with the flowers in the next great phase of organic life. But all
the other orders of insects are represented, and many of our
modern genera are fully evolved. The giant insects of the
Coal-forest, with their mixed patriarchal features, have given
place to more definite types. Swarms of dragon-flies, may-flies,
termites (with wings), crickets, and cockroaches, may be gathered
from the preserved remains. The beetles (Coleopters) have come on
the scene in the Triassic, and prospered exceedingly. In some
strata three-fourths of the insects are beetles, and as we find
that many of them are wood-eaters, we are not surprised. Flies
(Dipters) and ants (Hymenopters) also are found, and, although it
is useless to expect to find the intermediate forms of such frail
creatures, the record is of some evolutionary interest. The ants
are all winged. Apparently there is as yet none of the remarkable
division of labour which we find in the ants to-day, and we may
trust that some later period of change may throw light on its
origin.
Just as the growth of the forests--for the Mesozoic vegetation
has formed immense coal-beds in many parts of the world, even in
Yorkshire and Scotland--explains this great development of the
insects, they would in their turn supply a rich diet to the
smaller land animals and flying animals of the time. We shall see
this presently. Let us first glance at the advances among the
inhabitants of the seas.
The most important and stimulating event in the seas is the
arrival of the Ammonite. One branch of the early shell-fish, it
will be remembered, retained the head of its naked ancestor, and
lived at the open mouth of its shell, thus giving birth to the
Cephalopods. The first form was a long, straight, tapering shell,
sometimes several feet long. In the course of time new forms with
curved shells appeared, and began to displace the
straight-shelled. Then Cephalopods with close-coiled shells, like
the nautilus, came, and--such a shell being an obvious
advantage-- displaced the curved shells. In the Permian, we saw,
a new and more advanced type of the coiled-shell animal, the
Ammonite, made its appearance, and in the Triassic and Jurassic
it becomes the ogre or tyrant of the invertebrate world.
Sometimes an inch or less in diameter, it often attained a width
of three feet or more across the shell, at the aperture of which
would be a monstrous and voracious mouth.
The Ammonites are not merely interesting as extinct monsters of
the earth's Middle Ages, and stimulating terrors of the deep to
the animals on which they fed. They have an especial interest for
the evolutionist. The successive chambers which the animal adds,
as it grows, to the habitation of its youth, leave the earlier
chambers intact. By removing them in succession in the adult form
we find an illustration of the evolution of the elaborate shell
of the Jurassic Ammonite. It is an admirable testimony to the
validity of the embryonic law we have often quoted--that the
young animal is apt to reproduce the past stages of its
ancestry--that the order of the building of the shell in the late
Ammonite corresponds to the order we trace in its development in
the geological chronicle. About a thousand species of Ammonites
were developed in the Mesozoic, and none survived the Mesozoic.
Like the Trilobites of the Primary Era, like the contemporary
great reptiles on land, the Ammonites were an abortive growth,
enjoying their hour of supremacy until sterner conditions bade
them depart. The pretty nautilus is the only survivor to-day of
the vast Mesozoic population of coiled-shell Cephalopods.
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