Applying this scheme to the horse, it will be evident that as there is " one big digit ", consisting of four phalanges extending from the knee or hock to the toe, on the hoof-covered tip of which the horse stands, the animal must be classed with the perissodactyles or odd-toed, and also with the Solidungula or single-hoofed mammals.

Attached to the back of the one big digit of the horse, the leg-bone, cannon or shank bone as it is sometimes called, are the well-known splint-bones, one on each side, reaching from the knee or hock to a point about two-thirds of the length of the first of the phalanges. It can hardly be doubted that these splint-bones are the vestiges or representatives of the second and fourth digits seen in the remote ancestors of our horses. What has become of the first and fifth digits is a question to which no convincing answer has yet been given. Certain horny excrescences, termed carns or chestnuts, situated on the inner side of the legs above the knees and at the lower part of the hocks, and also the horny growths found at the back of the fetlock joints, partly or entirely concealed by the longhair which is usually abundant in that part, have been looked upon as the rudiments of the missing digits; but there are some facts connected with their situation in the limbs which do not support this view. Whatever may be their true place in the animal economy, these horny growths have always attracted attention, and much speculation has been indulged in as to their meaning. At the least it may be said of them that they serve to identify the members of the equine family, and to some extent aid in separating the various members of the group one from the other. In their typical form the chestnuts on the hind and fore extremities are characteristic of Equus caballus - the scientific name of the horse. Asses and zebras have them in a much-modified form on the fore limbs only. The excrescences (ergots) at the back of the fetlock are as in the horse.

The anatomical characters of the growths will be described more par-ticularly in connection with some other specialities of the horse when the general structure of the animal is considered.

At this point it will be convenient to pause for a moment to note the general character of the evidence which has been produced.

The preceding remarks have enabled us to ascertain with some exactness the place of the horse in nature, and we have further noted some of the more prominent special characters of the Equidae in their relations to the fossil remains of extinct animals in which those special features had a more perfectly-developed form, suggesting that in those animals they formed an actively useful and essential part of their organization.

At this early stage of the investigation it is not intended to suggest that the evidence which has already been advanced is in itself sufficient to prove that the horse is a descendant of some remote ungulate mammal which had five perfect digits instead of the " one big digit" by which it is now distinguished. On the contrary, many more facts have to be brought forward and carefully analysed before that proposition can be considered as proved.

Huxley, in his lectures on evolution, delivered in New York in 1876, observes that the occurrence of historical facts is said to be demonstrated when the evidence is of such a character as to render the assumption that they did not happen improbable in the highest degree. It is requisite, therefore, to consider the evidence bearing on the evolution of the horse, and it will render the subject all the more easy of comprehension if an attempt be made to explain what the word evolution is intended to express.

At the outset it may be remarked that the doctrine of evolution is not exactly new. " The great theory of evolution", writes Mr. Hutchinson in The Creatures of Other Days, " was first dimly suggested by Greek philosophers, such as Anaximander (b.c. 610), who may have derived the idea from Egyptian, Babylonian, or Hindu sources; then revived in a more scientific form by Lamarck last century. In recent years it has been placed on a truly scientific basis by the illustrious Charles Darwin, and is now generally accepted by naturalists. Indeed it is hard in these days to escape being an evolutionist, so abundant is the evidence in favour of the doctrine, especially that derived from a study of extinct animals."

Huxley writes in reference to evolution as the acting force in the past history of Nature, " that at any comparatively late period of past time. an imaginary spectator would have met with a state of things very similar to that which now obtains; but that the likeness of the past to the present would gradually become less and less, in proportion to the remoteness of his period of observation from the present day. Preceding the forms of life which now exist, the observer would see animals and plants not identical with them but like them, their differences increasing with their antiquity, and at the same time becoming simpler and simpler; until finally the world of life would present nothing but that undifferentiated protoplasmic matter, which, so far as our present knowledge goes, is the common foundation of all vital activity!" To all of which the reader, according to his views, may urge the series of objections which have from the first been formulated and overruled. How is it possible, it may be asked, that a mass of protoplasmic matter - a simple, jelly-like mass, giving hardly any evidence of life - can, under the influence of varying conditions of environment, become resolved into plants and animals, advancing steadily from the lowest forms to the highest? Clearly, the answer comes; the possibility cannot be disputed, the changes are going on perpetually under our eyes. Take the seed of a plant, or, better still, the ovum of an animal, and place it under favourable conditions, and the process of evolution begins and goes on to its completion. Structures are successively evolved without any interference from without, until a miniature man, or a lower animal, or a plant is formed. It is very interesting to observe that in the process of development, as Von Baer found, every organism in its earliest stages has the greatest number of characters in common with all other organisms in their earliest stages, and at a stage somewhat later, its structure is like the structures displayed at corresponding phases by a less extensive multitude of organisms. At each subsequent stage features are acquired which successively distinguish the embryo from groups of embryos which it previously resembled, thus step by step diminishing the class of embryos which it still resembles, and finally the class is narrowed to the species of which it is a member. The embryo of a bird or a rabbit has at one time in its development characters resembling those of the embryo of the fish - structures representing gill-clefts, for example. In the human embryo, it is only after exhibiting successive changes characteristic of the organization of lower animals that it at last assumes the form proper to man.