Popular Science Monthly/Volume 16/December 1879/Many-Toed Horses
WHEN Professor Huxley gave his lectures in New York, three years ago, on the evidences of evolution, he brought forward the genealogy of the horse as made out by recent fossil discoveries, and claimed that it was decisive in establishing the principle of descent, derivation, and development through the geological periods. There was a good deal of wise shaking of heads and shrugging of shoulders, at his presentation of the case, on the part of many who attended the lectures; and all who were perfectly ignorant of comparative anatomy and could not comprehend the course and force of the argument, were certain that the great biologist had for once made a total failure. No doubt if these critics had been questioned they would have readily pronounced the case closed for ever against evolution; but knowledge grows and evidence accumulates, and so it will be worth while to recall the subject, that we may appreciate some of the further points of illustration that have been made out since.
Professor Marsh, of Yale College, who has had this inquiry especially in hand, has made a short communication to "Silliman's Journal," on "Polydactyle (many-toed) Horses, Recent and Extinct," the substance of which we here reproduce.
It is stated that America is the original home of the horse, and that during the whole of Tertiary time, which the geologists divide into three periods—the Eocene, Miocene, and Pliocene—early, middle, and later—this continent was occupied with horse-like mammals of many and various forms. These all became extinct before the discovery of the country, but their abundant remains furnish the materials for marking out the genealogy of the horse in an almost unbroken succession of forms.
The study of fossils has shown that the oldest representatives of the horse on this continent all had many toes, and were of small size. In the course of development there was a gradual increase in size and a diminution in the number of toes, until the present type of horse was produced. The line of genealogy has been made out through seven successive stages, and the fossil proofs of its validity and completeness are all to be seen in the Yale Museum of Natural History. In vol. x. of "The Popular Science Monthly," page 295, the figures are given that illustrate the whole subject; we here simplify the representation by indicating the succession of changes that have taken place in the structure of the fore-foot of this series of quadrupeds (Fig. 1). All the facts go to show that the horse tribe is derived from an original ancestor having five toes on each foot, but this parent of the race has not yet been discovered. The oldest member of the group that has become known is the Eohippus, which had four well-developed toes and the rudiment of another on each fore-foot, and three toes behind. It was about as large as a fox, and appears in the lower Eocene or at the base of the Tertiary formation. It was discovered since Professor
Huxley's lectures were given, and since the diagrams we follow were made, and we therefore have no figure of it. The Orohippus, in the next higher division of the Eocene, resembled its predecessor in size, but had only four toes in front, as the diagram shows. The Mesohippus came later, was about as large as a sheep, and had three usable toes, and the splint of another, on each fore-foot. In the later Miohippus, the splint-bone is reduced to a short remnant. In the Pliocene above, a three-toed horse (Protohippus), about as large as a donkey, was abundant; and, still higher up, a near ally of the modern horse (Pliohippus) makes his appearance. The series is completed in the subsequent appearance of a true Equus, as large as the existing horse.
The horse has thus advanced in his development by getting rid of superfluous toes or digits; but, under the principle of reversion to an early ancestral type, to which it is now well understood that animals are liable in various ways, these suppressed splints or digits break out as extra hoofs. Professor Marsh says: "In addition to each main digit of the ordinary horse, the anatomist finds concealed beneath the skin two slender metapodial 'splint-bones,' which are evidently the remnants of two other toes originally possessed by the ancestor of the horse. It is an interesting fact that these splint-bones are sometimes quite fully developed, and may even support extra digits which are much shorter and smaller than the main foot. As these small hooflets are usually regarded as a serious detriment to the animal, they are generally removed from the colt soon after birth; but, in such cases, the enlarged splint-bones not unfrequently indicate in the adult their former existence. Numerous cases of extra digits in the horse have been recorded, and in nearly all of them a single lateral hooflet was present on one of the forelegs."
Professor Marsh states that the first recorded instance of extra digits in the horse known to him are two mentioned by George Simon Winter, in his famous book on horses, published at Nuremberg in 1703. One of the horses referred to, and figured in this work, was "eight-toed," having a small extra digit on the inside of each foot. Winter states that this horse was exhibited in Germany in 1663, and a portrait of it preserved in Cologne. His account was derived from a person who had examined the animal. The other horse described by Winter had a small hoof in the inside of each fore-foot; and this steed, Winter states, he had not only seen but ridden. Other instances of this phenomenon are referred to, on the authority of Geoffrey Saint-Hilaire, Owen, and Leidy.
Professor Marsh has described an interesting case of this reversion in the horse, which he has personally examined, and which is represented in Fig. 2. He says: "This animal was on exhibition in New Orleans in the spring of 1878, and Dr. Sanford E. Chaillé, of that city, first called the attention of the writer to it, and likewise sent a photograph from which the cut was made. This same horse was subsequently brought to the North, and a few days since was on exhibition at New Haven, Connecticut, where the writer examined him with some care. The animal is of small size, about ten years old, and is said to have been foaled in Cuba. He is known among showmen as the 'eight-footed Cuban horse.' With the exception of the extra digits he is well formed. The four main hoofs are of the ordinary form and size. The extra digits are all on the inside, and correspond to the index-finger of the human hand. They are less than half the size of the principal toes, and none of them reach the ground.
"Among the instances of recent polydactyle horses described to the writer by those who have seen them are two of special interest. One of these was a colt with three toes on one fore-foot, and two on the other. The animal recently died in Ohio. Another is a mare, raised in Indiana, and is still living, which is said to have three toes on each fore-foot, and a small extra digit on each hind-foot. In regard to the latter animal, the writer hopes soon to have more definite information.
"Besides the instances mentioned above of extra digits in place in the existing horse, there are many cases on record of true monstrosities, as, for example, additional feet or limbs attached to various portions of the body. Such deformities now admit of classification and explanation, but need not be considered in the present discussion."