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

BOOK 3

hoof. Not until some weeks after birth do the separate joint-surfaces disappear, and the three end joints coalesce with each other and with the elongated first joint to form the little buttons that tip the splint-bones (Fig. 125).

Thus the three-toed state occurs twice, in two different ways, in the history of horses. It occurred as a stage in the past history of the race; and it occurs as a stage in the development of every modern one-toed individual: the current stage recapitulates the three-toed past in its own embryo person.

All this in broad outline was known more than half a century ago, when isolated representatives of all our four main stages had already been described, and the general trend of horse evolution correctly deduced from them. But to-day we have more than the broad outline ; we are able to fill in the details. It is amazing how fine the detail has now become.

The amount of material at our disposal is enormous. In this single horse-stock, beginning with the little four-toed creatures from the Eocene (V A) and ending with the forms alive to-day, a total of over two hundred and sixty species have now been distinguished and named. It is, of course, true that in a continuously changing life-flow like this, species, genera, and families become merely arbitrary, since no sharp lines can be drawn. But even a continuous melody is divided up into bars: and the naming of two hundred and sixty species of fossil horses means roughly that we may chop up the continuously written record into two hundred and sixty pieces, making the difference between each piece and the next about equal to that found between forms recognized as species in an average abundant and variable family living to-day. Actually, however, in many cases the species and genera of fossil horses which were readily separated in early days have simply blended into each other as new material came to light. Sharp breaks in fossil history appear always to be the result of poverty of finds. Furthermore, not only are there these hundreds of types, but of many single types hundreds or even thousands of specimens have been unearthed. Occasionally the specimens are complete, but more often they are fragmentary, the skull (and especially the lower jaw) being perhaps most commonly found. All together, the specimens of evolving horses now in the museums of the world run into several tens of thousands. Even of the Lower Pliocene (V D 1) three-toed forms alone, at least ten thousand have been collected ; and though

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THE S@GIENCE OF LIFE

GCHAPTER 2

they come from many localities in Europe, North Africa, Asia Minor, India, Persia, Mongolia, China, and North America, all of them agree with one another in their testimony with regard to the evolution of teeth and feet.

There are two complications of the horse’s history. First, though the main trend is always on towards the modern horse, many of the fossil types represent side twigs which have died out sooner or later, leaving only the central branch to grow on to culmination. Second, the horses, being a very mobile species which can travel great distances rapidly, have a wide arena for their development. The earliest forms so far found are from the Western United States ; but from North America they soon invaded the Old World across a land connection where is now Behring Strait; and the living tide flowed back and forth between the Old World and the New, or was dammed back, according as this bridge emerged or was sunk under the waters; it invaded Africa, and flowed down into South America, when the Central American connection came into existence, in the late Miocene Period (V C 3). Thus, as climate changed and barriers were bridged, the various types of horse would move from place to place of this wide scene, so that sometimes quite new types suddenly appear in the local record—invaders which were evolved in some other locality where perhaps fossils have not yet been discovered. But in spite of these obscuring factors the story has been clearly worked out ; the fossils are so numerous that it can be construed without any doubt at all.

It will repay our trouble if we try to penetrate a little deeper into this representative chapter of life’s history. First, we may ask what could have been the reasons for this steady evolution in one direction. The answer appears to lie largely in a climatic change. The evidence of fossil plants makes it clear that during the Cenozoic, from Eocene (V A) to Pliocene (V D), the climate of the north temperate and_ subtropical regions became progressively drier, and that over much of North America and Eurasia forest gradually gave place to glades, and these to open plains, on which the newly evolved grasses flourished and spread. It was but natural that a few of the animals from the already crowded forests ventured out to try their luck in the new world that thus presented itself. It was a new, exacting life ; the new plants of the plains were wiry and hard to chew, and in a plain, since effective concealment is difficult, one has to