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.^ 



Tlie 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 Ecjuidse, comprising 

 the living horse, asses, and zebras, differ widely from all other 

 mammals in the peculiar structure of the feet, all of Avhich 

 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 oiu'selves, 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 i"uns 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 wi'ist. 

 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 ansAver to the joints of oiu* middle fingers, while the 

 hoof is simj^ly a gi'eatly enlarged and thickened nail. But if 



^ From a paper by Messrs. Scott and Osborne, "On the Origin and 

 Development of the Rhinoceros Group," read before the British Association 

 in 18S3, 



