not interlock, either with one another or with the
bones of the hand or foot below them. This, of course, would give a
weak foot, suited to slow progression over marshy ground--which, as we
have seen, was no doubt the origin of the mammalian plantigrade foot.
Here, for instance, to all intents and purposes, is a similar type of
foot, which belonged to a very early mammal, antecedent to the elephant
series, the horse series, the rhinoceros, the hog, and, in short, all
the known hoofed mammalia (Fig. 80). It was presumably an inhabitant of
swampy ground, slow in its movements, and low in its intelligence.

[Illustration: FIG. 80.--Fossil skeleton of _Phenacodus primavus_.
(After Cope.)]

But now, as we have seen, for more rapid progression on hard uneven
ground, a stronger and better jointed foot would be needed. Therefore we
find the bones of the wrist and ankle beginning to interlock, both among
themselves and also with those of the foot and hand immediately below
them. Such a stage of evolution is still apparent in the now existing
elephant. (See Fig. 81.)

[Illustration: FIG. 81.--Bones of the foot of four different forms
of the perissodactyl type, showing gradual reduction in the number
of digits, coupled with a greater consolidation of the bones above
the digits. The series reads from right to left. Drawn from nature
(_Brit. Mus._).]

Next, however, a still stronger foot was made by the still further
interlocking of the wrist and ankle bones, so that both the first and
second rows of them were thus fitted into each other, as well as into
the bones of the hand and foot beneath. This further modification is
clearly traceable in some of the earlier perissodactyls, and occurs in
the majority at the present time. Compare, for example, the greater
interlocking and consolidation of these small bones in the Rhinoceros as
contrasted with the Elephant (Fig. 81). Moreover, simultaneously with
these consolidating improvements in the mechanism of the wrist and ankle
joints, or possibly at a somewhat later period, a reduction in the
number of digits began to take place. This was a continuation of the
policy of consolidating the foot, analogous to the dropping out of the
sixth row of small bones in the paddle of _Baptanodon_. (Fig. 78.) In
the pentadactyl plantigrade foot of the early mammals, the first digit,
being the shortest, was the first to leave the ground, to dwindle,
and finally to disappea