not say anything
more about the plantigrade division. But the digitigrade modification
necessitated a change of structural plan, to the extent of raising the
wrist and ankle joints off the ground, so as to make the quadruped walk
on its fingers and toes. We meet with an interesting case of this
transition in the existing hare, which while at rest supports itself on
the whole hind foot after the manner of a plantigrade animal, but when
running does so upon the ends of its toes, after the manner of a
digitigrade animal.

It is of importance for us to note that this transition from the
original plantigrade to the more recent digitigrade type, has been
carried out on two slightly different plans in two different lines of
mammalian descent. The hoofed mammals--which are all digitigrade--are
sub-classified as artiodactyls and perissodactyls, i. e. even-toed and
odd-toed. Now, whether an animal has an even or an odd number of toes
may seem a curiously artificial distinction on which to found so
important a classification of the mammalian group. But if we look at the
matter from a less empirical and more intelligent point of view, we
shall see that the alternative of having an even or an odd number of
toes carries with it alternative consequences of a practically important
kind to any animal of the digitigrade type. For suppose an aboriginal
five-toed animal, walking on the ends of its five toes, to be called
upon to resign some of his toes. If he is left with an even number, it
must be two or four; and in either case the animal would gain the
firmest support by so disposing his toes as to admit of the axis of his
foot passing between an equal number of them--whether it be one or two
toes on each side. On the other hand, if our early mammal were called
upon to retain an odd number of toes, he would gain best support by
adjusting matters so that the axis of his foot should be coincident with
his middle toe, whether this were his only toe, or whether he had one on
either side of it. This consideration shows that the classification into
even-toed and odd-toed is not so artificial as it no doubt at first
sight appears. Let us, then, consider the stages in the evolution of
both these types of feet.

Going back to the reptile _Chelydra_, it will be observed that the axis
of the foot passes down the middle toe, which is therefore supported by
two toes on either side (Fig. 78). It may also be noticed that the wrist
or ankle bones do