In a
running attitude--the experiment may be tried--the weight of the
body is shifted from the flat sole of the foot, and thrown upon
the toes, especially the central toes. This indicates the line of
development of the Ungulates (hoofed animals) in the struggle of
the Tertiary Era. In the early Eocene we find the Condylarthra
(such as Phenacodus) with flat five-toed feet, and such a mixed
combination of characters that they "might serve very well for
the ancestors of all the later Ungulata" (Woodward). We then
presently find this generalised Ungulate branching into three
types, one of which seems to be a patriarchal tapir, the second
is regarded as a very remote ancestor of the horse, and the third
foreshadows the rhinoceros. The feet have now only three or four
toes; one or two of the side-toes have disappeared. This
evolution, however, follows two distinct lines. In one group of
these primitive Ungulates the main axis of the limb, or the
stress of the weight, passes through the middle toe. This group
becomes the Perissodactyla ("odd-toed" Ungulates) of the
zoologist, throwing out side-branches in the tapir and the
rhinoceros, and culminating in the one-toed horse.
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