CHAPTER I.

THE AGE OF REPTILES.

ITS ANTIQUITY, DURATION AND SIGNIFICANCE IN GEOLOGIC HISTORY.

Palaeontology deals with the History of Life. Its time is measured in
geologic epochs and periods, in millions of years instead of
centuries. Man, by this measure, is but a creature of yesterday--his
"forty centuries of civilization"[1] but a passing episode. It is by
no means easy for us to adjust our perspective to the immensely long
spaces of time involved in geological evolution. We are apt to think
of all these extinct animals merely as prehistoric--to imagine them
all living at the same time and contending with our cave-dwelling
ancestors for the mastery of the earth.

In order to understand the place of the Dinosaurs in world-history, we
must first get some idea of the length of geologic periods and the
immense space of time separating one extinct fauna from another.

_The Age of Man._ Prehistoric time, as it is commonly understood, is
the time when barbaric and savage tribes of men inhabited the world
but before civilization began, and earlier than the written records on
which history is based. This corresponds roughly to the Pleistocene
epoch of geology; it is included along with the much shorter time
during which civilization has existed, in the latest and shortest of
the geological periods, the Quaternary. It was the age of the mammoth
and the mastodon, the megatherium and Irish deer and of other
quadrupeds large and small which are now extinct; but most of its
animals were the same species as now exist. It was marked by the great
episode of the Ice Age, when considerable parts of the earth's surface
were buried under immense accumulations of ice, remnants of which are
still with us in the icy covering of Greenland and Antarctica.

_The Age of Mammals._ Before this period was a very much longer
one--at least thirty times as long--during which modern quadrupeds
were slowly evolving from small and primitive ancestors into their
present variety of form and size. This is the Tertiary Period or Age
of Mammals. Through this long period we can trace step by step the
successive stages through which the ancestors of horses, camels,
elephants, rhinoceroses, etc., were gradually converted into their
present form in adaptation to their various habits and environment.
And with them were slowly evolved various kinds of quadrupeds whose
descendants do not now exist, the Titanotheres, Elotheres, Oreodonts,
etc., e