fitted for
rapid motion on a plain, but the foot itself is adapted to rough
mountain work as well, and it is to this advantage, in part, that
the Artiodactyls owe their present supremacy. The plantigrade
pentadactyl foot of the primitive Ungulate--and even the
perissodactyl foot that succeeded it--both belong to the past humid
period of the world's history. As the surface of the earth slowly
dried up, in the gradual desiccation still in progress, new types
of feet became a necessity, and the horse, antelope, and camel were
gradually developed, to meet the altered conditions.

The best instance of such progressive modifications in the case of
perissodactyl feet is furnished by the fossil pedigree of the existing
horse, because here, within the limits of the same continuous family
line, we have presented the entire series of modifications.

There are now known over thirty species of horse-like creatures,
beginning from the size of a fox, then progressively increasing in bulk,
and all standing in linear series in structure as in time. Confining
attention to the teeth and feet, it will be seen from the wood-cut on
page 189 that the former grow progressively longer in their sockets, and
also more complex in the patterns of their crowns. On the other hand,
the latter exhibit a gradual diminution of their lateral toes, together
with a gradual strengthening of the middle one. (See Fig. 83.) So that
in the particular case of the horse-ancestry we have a practically
complete chain of what only a few years ago were "missing links." And
this now practically completed chain shows us the entire history of what
happens to be the most peculiar, or highly specialized, limb in the
whole mammalian class--namely, that of the existing horse. Of the other
two wood-cuts, the former (Fig. 84) shows the skeleton of a very early
and highly generalized ancestor, while the other is a partial
restoration of a much more recent and specialized one. (Fig. 85.)

[Illustration: FIG. 83.--Feet and teeth in fossil pedigree of the
Horse. (After Marsh.) _a_, bones of the fore-foot; _b_, bones of the
hind-foot; _c_, radius and ulna; _d_, tibia and fibula; _e_, roots
of a tooth; _f_ and _g_, crowns of upper and lower molar teeth.]

[Illustration: FIG. 84.--_Palaeotherium_. (Lower Tertiary of Paris
Basin.)]

[Illustration: FIG. 85.--_Hipparion_. (New World Pliocene.)]

On the other