The Evolution of the Horse. 
115 
Artiodactyle ; while the horse, the tapir, and the rhinoceros, 
are formed upon the other — the Perissodactyle type, to which, 
therefore, we may here solely confine our attention. 
The remains of animals to which it is possible to trace back 
these three existing forms, through a series of gradual modifica- 
tions, without any great break, are found in abundance in the 
lower strata of the great lacustrine formations assigned to the 
Eocene period, spread over considerable portions of the present 
territories of New Mexico, "Wyoming, and Utah, in North 
America. Similar animals also existed in other parts of the 
world ; but in Europe, the hitherto-discovered fragments which 
prove their existence are in a less complete and satisfactory con- 
dition for investigation. After giving a summary of what was 
then known of the ancestry of the horse, as revealed by palreon- 
tological evidence, Professor Huxley wrote in 1877: — "The 
knowledge we now possess justifies us completely in the antici- 
pation that, when the still lower Eocene deposits, and those that 
belong to the Cretaceous epoch, have yielded up their remains of 
ancestral equine animals, we shall find, first, a form with four 
complete toes, and a rudiment of the innermost, or first digit in 
front, with probably a rudiment of the fifth digit in the hind 
foot; while, in still older forms, the series of digits will be more 
and more complete, until we come to the five-toed animals, in 
which, if the doctrine of evolution is well founded, the whole 
series must have taken origin.'' 
This anticipation has been completely verified by the dis- 
covery, among others, of an animal called Phenacodus in the 
oldest of the Tertiary formations of North America. Skeletons 
of this animal in a remarkably complete state of preservation 
have been described by Professor Cope. They show an extremely 
primitive or generalised structure — as much so, indeed, as in any 
known mammal. The teeth had very short crowns, with tuber- 
culated surfaces, and were adapted for an omnivorous diet. There 
were five toes on each limb, which carried hoofs at the ex- 
tremities. The head was very small, and the size and shape of 
the brain-cavity indicated a low state of development of the 
organ it contained. As many as nine different species have been 
already described, varying in size from a bulldog to a leopard 
or sheep. The next very distinct stage of evolution leads us to 
a small animal, not larger than a fox, which once inhabited our 
own island, as the first indication of its existence was a skull 
discovered, in 1839, in the London clay of Heme Bay, to which 
Owen gave the name of Hyrucotherium. In this and many 
allied forms the number of toes was reduced to four on the 
front foot, and three on the hind foct, No. L being absent in 
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