tiary we find a forest, with trees
that shed their leaves, interspersed with glades, in which already the
grasses were beginning to be developed. This state of affairs had
existed but for a comparatively short time, geologically speaking. It
had come only in the latter part of the preceding era. Lake and swamp,
meadow and forest intermingled to make a rich and varied scene. Slowly
the land toward the western side of North America lifted itself into
plateau and mountain range. Slowly the westerly winds began to be cut
off by the barriers thus raised across their path. As they swept over
the plateau and down into the eastern plain their moisture came to be
diminished. Gradually a very different state of affairs set in. The
ground became harder, the forest became sparser, the plants became
higher and firmer, the grasses tougher and more wiry, and, by the time
the Quaternary arrived, a condition probably even drier than that of
to-day existed over our western highlands. Throughout this long
change, spread over millions of years, a creature which has become our
horse steadily persisted and steadily advanced. Side lines developed
which finally disappeared, but the main line kept on, and when the
Quaternary came the horse arrived with it. Many of the skeletons in
this series were known before it was realized what they were. As time
went on and intermediate forms were found, it became possible to
recognize these as ancestors of the horse and to assign them their
proper position in the family tree.

[Illustration: THE EVOLUTION OF THE HORSE'S FOOT

_After H. F. Osborne and Charles R. Knight. By permission of the
American Museum of Natural History._]

The earliest of the forerunners of the horse with which we are
acquainted would certainly not be recognized as such by any but the
most careful student of animals, if we could see him to-day. He stood
not higher than a fox-terrier dog, though his shape was very
different. But he would probably be more likely to be classed with the
dog than with the horse by the hasty observer, for he walked with four
toes of each foot upon the ground as the dog does to-day. Like the
dog, he had hanging at the inner side of his front foot a little
useless toe. He was long in body, comparatively short of leg, a little
long of head and neck, and distinctly long of tail. His grinding teeth
had points on them not unlike a pig's, and possessed no apparent
resemblance to the wonderful curved and ridged