dern carcharodon, who runs (as I have before
noted) to forty feet in length, is a very respectable monster indeed, as
times go; and his huge snapping teeth, which measure nearly two inches
long by one and a half broad, would disdain to make two bites of the
able-bodied British seaman. But the naturalists of the 'Challenger'
expedition dredged up in numbers from the ooze of the Pacific similar
teeth, five inches long by four wide, so that the sharks to which they
originally belonged must, by parity of reasoning, have measured nearly a
hundred feet in length. This, no doubt, beats our biggest existing
shark, the rhinodon, by some thirty feet. Still, the ooze of the Pacific
is a quite recent or almost modern deposit, which is even now being
accumulated on the sea bottom, and there would be really nothing
astonishing in the discovery that some representatives of these colossal
carcharodons are to this day swimming about at their lordly leisure
among the coral reefs of the South Sea Islands. That very cautious
naturalist, Dr. Guenther, of the British Museum, contents himself indeed
by merely saying: 'As we have no record of living individuals of that
bulk having been observed, the gigantic species to which these teeth
belonged must probably have become extinct within a comparatively recent
period.'

If these things are so, the question naturally suggests itself: Why
should certain types of animals have attained their greatest size at
certain different epochs, and been replaced at others by equally big
animals of wholly unlike sorts? The answer, I believe, is simply this:
Because there is not room and food in the world at any one time for more
than a certain relatively small number of gigantic species. Each great
group of animals has had successively its rise, its zenith, its
decadence, and its dotage; each at the period of its highest development
has produced a considerable number of colossal forms; each has been
supplanted in due time by higher groups of totally different structure,
which have killed off their predecessors, not indeed by actual stress of
battle, but by irresistible competition for food and prey. The great
saurians were thus succeeded by the great mammals, just as the great
mammals are themselves in turn being ousted, from the land at least, by
the human species.

Let us look briefly at the succession of big animals in the world, so
far as we can follow it from the mutilated and fragmentary record of the
g