It is perhaps well to emphasize the need of correct interpretation, for
there are no bridges on the paths of palaeontology, and as we go back,
more than one great gap occurs between series of strata, marking periods
of intervening time which there is no means of measuring, but during
which we know that the progress of change in the animals then living
never ceased. When such a break is reached, the course of phylogeny is
like picking up an interrupted trail, with the additional complication
that the one we find is never quite like the one we left, and it is in
such conditions that the systematist must apply his knowledge of the
general progressive tendencies through the ages of change, to the
determination of the particular changes he should expect to find in the
special case before him, and so be enabled 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.