South America, and India. We can imagine herds of these
creatures from fifty to eighty feet in length, with limbs and gait
analogous to those of gigantic elephants, but with bodies extending
through the long, flexible, and tapering necks into the diminutive
heads, and reaching back into the equally long and still more tapering
tails. The four or five varieties which existed together were each
fitted to some special mode of life; some living more exclusively on
land, others for longer periods in the water.

The competition for existence was not only with the great carnivorous
dinosaurs, but with other kinds of herbivorous dinosaurs (the
iguanodonts), which had much smaller bodies to sustain and a much
superior tooth mechanism for the taking of food.

The cutting off of this giant dinosaur dynasty was nearly if not quite
simultaneous the world over. The explanation which is deducible from
similar catastrophes to other large types of animals is that a very
large frame, with a limited and specialized set of teeth fitted only
to a certain special food, is a dangerous combination of characters.
Such a monster organism is no longer adaptable; any serious change of
conditions which would tend to eliminate the special food would also
eliminate these great animals as a necessary consequence.

[Illustration: Fig. 46.--Badlands on the Red Deer River in
Alberta. This region is the richest known collecting ground for
cretacic dinosaurs.]

There is an entirely different class of explanations, however, to be
considered, which are consistent both with the continued fitness of
structure of the giant dinosaurs themselves and with the survival of
their especial food; such, for example, as the introduction of a _new
enemy_ more deadly even than the great carnivorous dinosaurs. Among
such theories the most ingenious is that of the late Professor Cope,
who suggested that some of the small, inoffensive, and inconspicuous
forms of Jurassic mammals, of the size of the shrew and the hedgehog,
contracted the habit of seeking out the nests of these dinosaurs,
gnawing through the shells of their eggs, and thus destroying the
young. The appearance, or evolution, of any egg-destroying animals,
whether reptiles or mammals, which could attack this great race at
such a defenseless point would be rapidly followed by its extinction.
We must accordingly be on the alert for all possible theories of
extinction; and these theories themselves will fal