Of course, these early mammals are known to us only by their fossil and
mostly fragmentary skeletons, but it may be said that at least in the
ungulate line, the successive geological periods show steady structural
progression in certain directions. Of great importance are a decrease in
the number of functional digits; a gradual elevation of the heel, so
that their modern descendants walk on the tips of their toes, instead of
on the whole sole; a constant tendency to the development of deeply
grooved and interlocked joints in place of shallow bearing surfaces; and
to a complex pattern of the molar crowns instead of the simple type
mentioned. To this may be added as the most important factor of all in
survival, that these changes have progressed together with an increase
in the size of the brain and in the convolutions of its outer layer.

The _Condylarthra_ seem to have gone out of existence before the
time of the middle Eocene, but before this they had become separated
into the two great divisions of odd-toed and even-toed ungulates, into
which all truly hoofed beasts now living fall.

The first group (_Perissodactyla_) has always one or three toes
functionally developed, either the third, or third, second and fourth,
the two others having entirely disappeared, except for a remnant of the
fifth in the forefoot of tapirs. They have retained some at least of the
upper incisor teeth, and, except in some rhinoceroses, the canines are
also left; the molars and premolars are practically alike in all recent
species, and in all of which we know the soft parts, the stomach has but
one compartment, and there is an enormous caecum. It is probable that
they took rise earlier than their split-footed relations, and their
Tertiary remains are far more numerous, but their tendency is toward
disappearance, and among existing mammals they are represented only by