The science of life : fully illustrated in tone and line and including many diagrams
THE EVIDENCE
other animals have quite similar teeth. The family now comprises one genus only, called Equus and including seven speciestwo Asiatic wild asses (the onager and kiang), one African wild ass, the little Przevalsky’s horse from Asia, and three kinds of zebra from Africa. A fourth kind of zebra, the quagga, has recently been exterminated. In addition, there are, of course, the many varieties of domestic horse and donkey, brought into being by man’s selec-
tive breeding. Now all these wild
horses and asses and zebras live in
much the same way. They run
swiftly over hard, level plains (wild
asses in the Mongolian desert have
been timed doing their forty miles
per hour), and they feed on a food
that is difficult to chew—on the
tough leaves and stems of grasses,
which are often hardened by a
certain amount of flinty matter.
And corresponding with this hard-
ness of ground and of food, we find
special provisions in their feet and
teeth.
The single toe of a horse’s foot corresponds to the third or middle toe of the more ordinary five-toed foot. Only the last joint ofthe toe touches the ground ; the hoof in which this last joint is encased is the exact equivalent of an overgrown toenail. The horse’s wrist and ankle are far above the ground, forming the joints commonly called ‘“‘ knee ” and “hock’’; the true knee is what is styled the stifle. The region corresponding to our palm or sole contains the single elongated cannon-bone. . But the other fingers or toes are not completely absent, for attached to the hinder angles of this cannonbone are two little splint-bones : and that these are the remains of the second and fourth fingers and toes is amply proved not merely by their position, and by the fact that one or both of them occasionally develop the missing joints and a miniature hoof, but also, as we shall soon see, by their development in the embryo.
The horse’s limb, then, is a specially modified limb ; it is a limb in which one toe is enormous and strong, and in which the others have dwindled more or less completely away. It is a limb devoted to a special function. On open, grassy plains the best means of escape from enemies lies in speed ; and it is for speed on comparatively level
ee
OF EHE ROCKS
and hard ground that the legs of the horse are suited. Everything else has been sacrificed to that. The elongation of the actual footregion gives a better leverage; the concentration of all the limb-muscles in the upper part of the legs allows for rapid swing ; a limb which consists of a single pillar, jointed So as to move only fore-and-afi, transmitting all the weight downwards to a single expanded hoof, is stronger than one which can
wer
fy
YZ
Fig. 123. Stages in the evolution of the horse’s hoof; a
series of left fore-feet.
On the right, Eohippus with three of its four toes visible—the fifth, fourth, and third. Next to it, Mesohippus with fifth digit reduced to a_splint-bone, i ; ; Merychippus with fifth digit small and lifted off the ground, and enlarged central digit. big enue hoof, fifth digit quite disappeared, and second and fourth
and considerable lengthening of the toes. Then
On the left, the modern horse, Equus, with
digits reduced to splint-bones.
be moved in all directions, or than one in which the lower arm or leg contains two bones (as in ourselves), and there is less “give” than in a limb ending in several ce Accordingly the horse, though it can only execute a very few kinds of movements with its limb, though it sinks in soft ground owing to lack of spreading toes, though it is not well adapted to broken country, triumphs in speed on dry and rolling plains (Fig. 123). Incidentally, length of leg makes length 203