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

BOOK 3

insensibly into some of the Merychippus forms, first emerging distinctly towards the very end of the Miocene (V C 3) and continuing through the Lower Pliocene (V D 1). As regards feet, Pliohippus bridges the gap between Main Stages II] and IV, some of its seventeen species possessing tiny but perfect side toes complete with miniature but useless hoofs, while in others they were reduced to single splints as in modern horses. ‘The chief point of progress again concerns the teeth. In earlier genera, the molars led the way ; then their more elaborate pattern was gradually adopted by the premolars too. However, this applied to the permanent premolars ; their milk-teeth predecessors in the colt were always of simpler pattern. But in Pliohippus the elaborate pattern has been thrown back into the milk-tooth stage, and from now onwards all the grinders at every stage of life share equally in every advance made in the grinding machinery. The ninth genus is Plesippus, which comes definitely into the last of our four main stages. So far, only one species has been discovered, from the Upper Pliocene (V D 3), but of this one species the anatomy is known

THE SCIENCE OF LIFE

CHAPTER 2

in full detail. These creatures were very much like a smallish horse, but the hoofs, as in Pliohippus, were still much smaller than in a modern horse, and the teeth show but little advance on those of Pliohippus. The splint-bones representing the outer toes are very interesting, for they are longer than those of living horses and more expanded at the tip. The fifth digit of the fore-foot, which, as we saw, became useless in Mesohippus, hung on for a long time as a remnant, and was apparently loth to disappear, for it is still represented in Plesippus by a tiny nodule of bone. Indeed, in living horses, though usually wholly absent, it is still to be found in a few individuals as a still smaller nodule. The shape of the skull is like that of our modern horses.

From this type to Equus, the true horse, is but a small step, which was taken at the turn from Pliocene to Pleistocene (V E). Even among the various species of Equus, however, evolution can be seen at work, for many of the earlier species are both smaller and more primitive in tooth-pattern than any existing modern horses. On the other hand, one or two recently extinct forms consider-

RECENT(VF)) PLEISTOCENE | ¥ EOCEN E |.OLIGOCENE | MIOCENE] PLIQCENE ny somilsnyerrs 70 (WAS), 30 (VB) 20 | (VC) 1 (VD) AVE {= > / SE RHINOCEROSES > - TAPIRS > /) . FRANCA ALA (OS) fe a Zs > 7 Ry Pace gs F We LB ST “4 cnet fZ j Zz i Lf x ys Sy oo S LANE £ EWE 2 fee Z S| Fo LE ; = z . LLE | i /, L— - } bp c cel aa L-f == cnOrien ae MESOHIPPUS PARAHIPPUS~/P 1GnipPuSs 7 x roots === ==> -~SEOHIPPUS EPIHIPPUS MIOHIPPUS| MERY IS —\ PLESIPPUS = Ancestral. KES R ey S = One-toed>~- o Ungulates

CHALICOTHERES —>

— TITANOTHERES > —>

S Genera :— @) @) (3) @)

Main stages 1 Four-toed horses

Three-toed; all toes used \Three-foed;ouly central

IL TW

One- toed

toe uséd

Fig. 126.

The different geological periods are all re presented onthe same time-scale, except for the Pleistocene and. Recent Periods,

The Evolution of Horses.

whose scales have been magnified five and twenty-five times respectively. In the early Eocene the various stocks of odd-toed

Ungulates are scarcely to be distinguished, but soon the separate stocks (Rhinoceroses, Tapirs, Horses, etc.) diverge and

become markedly distinct. The horse stock alone is here followed throughout. The names of its ten successive genera mentioned

in the text are given, with (below) the numbers corresponding to them. Below this again the duration of the four

main stages is indicated. The initial stock is all the time throwing off side branches which become extinct ; most are short and unimportant, but some produced distinet bypes of horses, such as Hypohippus and Hipparion.

208