384 
DARWINISM 
CHAP. 
number of large hornless rhinoceroses were developed — 
they are found in the Upper Miocene, Pliocene, and Post- 
Pliocene formations—and then became extinct. The true 
rhinoceroses have three toes on all the feet. 1 
The Pedigree of the Horse Tribe. 
Yet more remarkable is the evidence afforded by the 
ancestral forms of the horse tribe which have been discovered 
in the American tertiaries. The family Equiche, comprising 
the living horse, asses, and zebras, differ widely from all other 
mammals in the peculiar structure of the feet, all of which 
terminate in a single large toe forming the hoof. They have 
forty teeth, the molars being formed of hard and soft material 
in crescentic folds, so as to be a powerful agent in grinding 
up hard grasses and other vegetable food. The former peculi¬ 
arities depend upon modifications of the skeleton, which have 
been thus described by Professor Huxley :— 
“ Let us turn in the first place to the fore limb. In most 
quadrupeds, as in ourselves, the fore-arm contains distinct 
bones, called the radius and the ulna. The corresponding 
region in the horse seems at first to possess but one bone. 
Careful observation, however, enables us to distinguish in this 
bone a part which clearly answers to the upper end of the 
ulna. This is closely united with the chief mass of the bone 
which represents the radius, and runs out into a slender shaft, 
which may be traced for some distance downwards upon the 
back of the radius, and then in most cases thins out and 
vanishes. It takes still more trouble to make sure of what is 
nevertheless the fact, that a small part of the lower end of the 
bone of a horse’s fore-arm, which is only distinct in a very 
young foal, is really the lower extremity of the ulna. 
“What is commonly called the knee of a horse is its wrist. 
The ‘ cannon bone ’ answers to the middle bone of the five 
metacarpal bones which support the palm of the hand in our¬ 
selves. The pastern, coronary, and coffin bones of veterin¬ 
arians answer to the joints of our middle fingers, while the 
hoof is simply a greatly enlarged and thickened nail. But if 
1 From a paper by Messrs. Scott and Osborne, “ On the Origin and 
Development of the Rhinoceros Group,” read before the British Association 
in 1883. 
