to recognize the footprints
he is in search of. The genius to do this has been given to few, but in
their hands the results have often been brilliant.

Back in the very earliest Tertiary deposits, and in all certainty even
earlier, a group of comparatively small mammals was extensively spread
through America, and apparently less widely in Europe, characterized by
a primitive form of foot structure, each of which had five complete
digits, the whole sole being placed upon the ground, as in the animals
we call plantigrade. The grinding surfaces of their molar teeth were
also primitive, bearing none of the complicated, curved crests and
ridges possessed by present ruminants, but instead they had conical
cusps, usually not more than three to a tooth; this tritubercular style
of molar crown being about the earliest known in true mammals.

In the opinion of many palaeontologists, the ancestors of the present
hoofed beasts, or ungulates, were contained among these
_Condylarthra_, as they were named by Prof. Cope.

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