hand, progressive modifications of the artiodactyl feet may
be traced geologically up to the different stages presented by living
ruminants, in some of which it has proceeded further than in others. For
instance, if we compare the pig, the deer, and the camel (Fig. 82), we
immediately perceive that the dwindling of the two rudimentary digits
has proceeded much further in the case of the deer than in that of the
pig, and yet not so far as in that of the camel, seeing that here they
have wholly disappeared. Moreover, complementary differences are to be
observed in the degree of consolidation presented by the two useful
digits. For while in the pig the two foot-bones are still clearly
distinguishable throughout their entire length, in the deer, and still
more in the camel, their union is more complete, so that they go to
constitute a single bone, whose double or compound character is
indicated externally only by a slight bifurcation at the base.
Nevertheless, if we examine the state of matters in the unborn young of
these animals, we find that the two bones in question are still
separated throughout their length, and thus precisely resemble what used
to be their permanent condition in some of the now fossil species of
hoofed mammalia.

Turning next from bones of the limb to other parts of the mammalian
skeleton, let us briefly consider the evidence of evolution that is here
likewise presented by the vertebral column, the skull, and the teeth.

As regards the vertebral column, if we examine this structure in any of
the existing hoofed animals, we find that the bony processes called
zygapophyses, which belong to each of the constituent vertebrae, are so
arranged that the anterior pair belonging to each vertebra interlocks
with the posterior pair belonging to the next vertebra. In this way the
whole series of vertebrae are connected together in the form of a chain,
which, while admitting of considerable movement laterally, is everywhere
guarded against dislocation. But if we examine the skeletons of any
ungulates from the lower Eocene deposits, we find that in no case is
there any such arrangement to secure interlocking. In all the hoofed
mammals of this period the zygapophyses are flat. Now, from this flat
condition to the present condition of full interlocking we obtain a
complete series of connecting links. In the middle Miocene period we
find a group of hoofed animals in which the articulation begins by a
slight roundin