The science of life : fully illustrated in tone and line and including many diagrams

THE EVIDENCE

only in details of proportion. grass-plain animals.

In the stage before this there are no onehoofed horses. Instead we find smaller creatures, of obviously horse-like type, but with three hoofs on each foot ; the two outer hoofs, however, are small and must have been useless in running, since they did not reach the ground, but hung in the air like the dewclaws of deer and other animals. The teeth had a less elaborate grinding system, and were much shorter. Fossil bones of theseshall we call them the fathers of the horses ? —are not uncommon in China, and (with others) are dug up to be sold to apothecaries as “* dragons * teeth,” that essential ingredient in the Chinese phar-

They are all

macopeeia. Professor Watson tells us that even the Chinese

labourers employed to dig them up recognize the skulls as like those of their donkeys.

In the next, and still older, assemblage of forms, the grandparents, so to speak, the ancestral horses were no larger than a large dog or a small Shetland pony. They also were three-toed ; but all three hoofs touched the ground, and in addition they possessed on the forefoot the trace of a fourth toe, in the form of a little splint against the cannonbone; the toothpattern again was less elaborate, the whole tooth shorter, and there was no trace of the cement which in all later forms filled up the valleys between the ridges of enamel and dentine, and so ensured a flat grinding surface throughout. Yet one can easily recognize the skeletons even of this stage as those of horses—three-toed and rather lumpish horses, but horses.

Finally, in the earliest stage, the far ancestors, in which we can still definitely detect the tendencies which culminated in the modern horse, none of the animals were bigger than a medium-sized terrier ; there were four little hoofs on the fore-foot and three on the hind ; sometimes the hind-foot also showed two splint-bones representing the missing first

Fig. 125. (A) The skeleton of the limb of an embryo horse six weeks old, showing three toes. ] Sor eight weeks old, showing the side toes much reduced relative io the middle toe, on which

the hoof is forming. to splints.

adult horses.

OF THE ROCKS

and fifth digits ; the teeth were very short, with only indications of the system of grinding ridges, and the premolars were not so large nor of so complicated a pattern as the molars.

Besides the teeth and toes, other characters, too, show steady parallel changes as we go back through time. The earlier forms had shorter necks and faces, less tightly fitted wrists and ankles, two separate bones instead of one in the lower arm and leg. In a word, as we go back we find horses less and less efficiently adapted for swift running and for grinding hard vegetable food, and more and more like the other generalized mammals of the early Eocene (V A 1).

The existence of these three-toed horses

The modern horse recapitulates its own evolution.

(B) The

(C) The same in a five-months embryo, showing side toes reduced (D) The end of a side toe of (C), much enlarged, showing its rudimentary hoof. (E) The end of a side toe at birth ; the various bones are still separate, not Joined as in : (Modified from drawings by Professor Cossar Ewart.)

in the past acquires a double significance when we remember that every individual one-toed horse of to-day actually passes through a three-toed stage in its embryonic development. In a six-weeks embryo the limbs are short; two separate bones are present to make the skeleton of the forearm ; the wrist and foot are short, and the middle toe is flanked by two other smaller but perfectly formed toes, each complete with the same number of joints as the centre toe. At eight weeks the two side toes are on the way to become mere splints, but they still show the full number of joints. ‘They eventually develop a cross between a nail and a hoof at their tip, and this later grows into a regular

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