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

THE EVIDENCE

run fast. Some of the adventurers chewed well enough and fled fast enough to survive ; the pursuing carnivores had yet to achieve the speed of the wolf. The earliest four-toed horses were forest-dwellers ; they must have lived as their closest surviving relations the tapirs live to-day, moving slowly and warily over soft ground and eating comparatively soft vegetation ; but the three-toed forms had for the most part taken to the grazing life of the plains. The progressive spread of the plains during the rest of the Cenozoic up to the Glacial Epoch (V E) put a premium upon continued improvement in the same direction. Thus the early horses increased and flourished. Their more conservative forest-dwelling cousins dwindled in numbers as their range was restricted, and in most cases became altogether extinct, while the primitive wolf and the primitive horse were teaching each other speed.

The various stages of this main line of evolution have been classified into ten genera, of which we append the following thumbnail sketches. Number one is Eohippus from the American Lower Eocene (V A 1), the earliest known four-toed horse ; of this genus thirteen species are now known, ranging in size from a cat to a medium-sized terrier. It represents the more primitive type of the earliest of our four main stages, still with rudiments of the first and fifth toes on the hind-foot. There are also three or four European species of the same age and general character, but with even. more primitive teeth, which have been given the name of Hyracotherium. Orohippus comes second, from the Middle Eocene (V A 32), with ten known species. This lacks the tudimentary hind-toes, and its premolar teeth gradually become more molar-like as We pass up in the rock layers from species to species. In both these types the lower arm and leg contained two separate bones, and the fore-limb could still be twisted and turned about in a way impossible for the modern horse. Number three, Epihippus, with only two species hitherto described, is from the Upper Eocene (V Ag). It is still four-toed, but the fourth toe is smaller, the central toes larger, and two of the three premolar teeth are just like the true grinders. There is no real break between it and Orohippus.

That closes our Main Stage I. Mesohippus is the first type of Main Stage II, the horses with three usable toes. Between it and Epihippus there is a slight gap, during which the final stages in the reduction of the

fourth front toe to the merest splint must have

OF THE ROCKS

been run through, and during which there was also a considerable increase in size. Thousands of specimens of the genus have been found, the earliest from the Lower Oligocene (V B 1), but running on through Middle into Upper ; these can be separated mto no less than eighteen species. All three of the actively-grinding premolars are now like the molars, and in the fore-arm and lower leg one of the two bones js enlarged to take nearly all the weight, while the other is on the way to disappearance.

The fifth genus is called Miohippus. It is first found in the Upper Oligocene (V B 3) and is scarcely to be separated from Mesohippus, save by its rather larger size (up to that of a sheep) and the betier mechanical construction of the skeleton of ils wrist and ankle. It also is abundant and variable, with seventeen known species. Parahippus, first found in the next higher rock formation, the Lower Miocene, is a real connecting link between Main Stage II and Main Stage III. The earlier of the eighteen species so far discovered are on the whole very like Miohippus, while some of the later forms have the outer two hoofs and digits so much reduced as to be useless. The teeth, too, are interesting ; the cement filling appears for the first time, but only in some of the later species is it anything but a very thin coating.

Parahippus runs on right through the Miocene : but already by the Middle Miocene (V C2) a more progressive type had appeared, which is called Merychippus. This type, abundant and wide-ranging, with some twenty-five species, dcfinitely initiates our Main Stage III, for its side toes never touch the ground. But it makes a greater advance in the depth of the teeth. In the earliest species of Merychippus the grinders are only as high as wide; in some of the latest, the height is two-and-a-half times the width. The cement is always abundant, though it was not deposited until just before the tooth emerged from its bone socket, instead of some time before, as in the living horse. ‘The ulna, the dwindling second bone in the lower fore-limb, is a separate splint in colts of Merychippus, but in the adults js fused with the main bone or radius, as it is from birth onwards in modern horses : here we get a second glimpse of recapitulation, that widespread phenomenon of which the three-toed embryos of modern horses have already given us an example. Much increase in size also took place within the group before it faded out in the early Pliocene (V D 1), the later forms being often as big as small ponies.

Our eighth genus, Pliohippus, grades back

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