Throughout the
long lapse of the secondary ages, across the lias, the oolite, the
wealden, and the chalk, we find the mammalian race slowly developing
into opossums and kangaroos, such as still inhabit the isolated and
antiquated continent of Australia. Gathering strength all the time for
the coming contest, increasing constantly in size of brain and keenness
of intelligence, the true mammals were able at last, towards the close
of the secondary ages, to enter the lists boldly against the gigantic
saurians. With the dawn of the tertiary period, the reign of the
reptiles begins to wane, and the reign of the mammals to set in at last
in real earnest. In place of the ichthyosaurs we get the huge cetaceans;
in place of the deinosaurs we get the mammoth and the mastodon; in place
of the dominant reptile groups we get the first precursors of man
himself.
The history of the great birds has been somewhat more singular. Unlike
the other main vertebrate classes, the birds (as if on purpose to
contradict the proverb) seem never yet to have had their day.
Unfortunately for them, or at least for their chance of producing
colossal species, their evolution went on side by side, apparently, with
that of the still more intelligent and more powerful mammals; so that,
wherever the mammalian type had once firmly established itself, the
birds were compelled to limit their aspirations to a very modest and
humble standard. Terrestrial mammals, however, cannot cross the sea; so
in isolated regions, such as New Zealand and Madagascar, the birds had
things all their own way.
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