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

THE EVIDENCE

ably exceeded any living wild species in size, and were probably even larger than our domestic draught-horses; their size is perhaps to be correlated with their living in an interglacial period when the world was warmer and richer in vegetation than it is nowadays. As many as forty-five fossil species have been unearthed and described, so that less than a sixth of the total known variety of the genus Equus is in existence to-day.

Thus has the change been brought about, from the tiny, furtive, forest-haunting, browsing Eohippus to the swift, strong, grazing Equus of the open country. But the study would not be complete without a mention of certain side branches which grew out, turning aside from the main trend of horse evolution and sooner or later came to nothing. We have this evolution of their extinction, step by step. .

The first of these branches started in the early Eocene (VA1). Some of the Old World, four-toed forms wandered off on their own, but became extinguished in the Lower Oligocene (V B 1) after advancing a certain way parallel with the main horse-stock. Paleotherium, three-toed but somewhat heavy and tapir-like, was the best known, but others were lighter and better adapted to swift running. Over twenty species are known to have been evolved in these sidelines ; what brought their career to a close is uncertain. .

The next divergent branch began at the close of the Miocene (V C). Its peculiarity consisted in its retaining three good toes on each foot, very much splayed out, and teeth less well adapted for grinding grass, but suitable for browsing leaves. While some species of Miohippus form a connecting bridge to Parahippus and the rest of the main horsestock, others connect equally insensibly with the base of this side-branch. The tendency, first revealed in them, runs its course through a couple of genera, culminating in the early Pliocene (V D1) in Hypohippus. In this branch, with at least ten known species, we can trace a progressive adaptation to forest life, the animals apparently eating more juicy food and supporting themselves on softer ground by the aid of their spreading toes. Here we have, so to speak, a line of quitters, of animals which shirked the more arduous life of the open plains that was the goal of their race and turned back into the woods. For a time they throve, but only for a time. It was not in this direction that the horse-stock was destined to win through.

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OF THE ROGKS

In the late Pliocene (V D 3) the horsestock invaded South America, and here was evolved the Hippidium branch, possessing extremely short legs and a strangely-constructed nose-region. These creatures were a group of three genera, of which only four species have as yet been unearthed, only extinguished within the last million years. Their short legs probably indicate that they were adapted to mountain life. The origin of this aberrant line is traceable through Protohippus, a group of thirteen somewhat earlier species.

The fourth and largest side-line is the Hipparion branch. It came off from Merychippus in the late Miocene and ran on to the close of the Pliocene, when it was extinguished, but not before it had given rise to some thirty-five species. It is interesting as retaining the two outer toes, though not touching the ground, long after the main-line had reduced them to splints ; while, on the other hand, some of its species improved their teeth beyond anything known even in present-day horses, the grinding pattern being more complex in some, the height of the teeth greater in others. It is likely that this increased specialization of teeth was an adaptation to a desert life, where the hard, dry vegetation needs more grinding. It may be that these horses fell out of the battle for life because of this too exclusive dependence on the special virtues of their teeth and were caught by a change of circumstances that made speed of greater importance. :

That is a condensed summary of the story of the horse and its ancestors and vanished cousins as we know it to-day. It is a tale of adventure and arduous conquest, of steady and successful adaptation of a race to new surroundings. But it is more interesting as a part of a vaster drama. It displays one streak of the process of Evolution very completely and convincingly. Step by step, variety by variety, the progressive changes can be traced. One can hardly say where one species ends and another begins. Doubtless our knowledge of fossil horses will be farther filled in and rounded off in the future, as new specimens turn up ; but new discoveries can do no more now than fill in a little gap here, correct a minor error there. The essential facts are already before us In their fullness. In one long gallery one might assemble all these stages. We have here in a crushing multitude of steadily progressing specimens just that complete, continuous exhibition of Evolution in action the Creationist has demanded. He is answered.

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