It was at one time the ambition of some breeders to have Shetland ponies as small as their remote three-toed Miocene ancestors. As a matter of fact, ponies smaller than some of the Miocene species have long existed in Shetland. Eric, though an average-sized pony, was at least a hand smaller than the late Miocene horse, Protohippus sejunctus, in which the front cannon-bones (fig. 9b) were 177 mm. long and 21 mm. wide—i.e., 34 mm. (1·37 inches) longer but 5 mm. narrower than in Eric; while Seedpearl (31·75 inches at the withers) and other still smaller living ponies have shorter limbs than the very ancient three-toed Mesohippus from the Badlands of South Dakota. But while some Shetland ponies are actually smaller and have relatively decidedly shorter legs than the horses which flourished long before man appeared on the scene, they never have three toes and their teeth are always decidedly longer if not more complex than in the most advanced Miocene species. Hence, as already said, though in Shetland ponies there is evidence of arrested growth, there is no evidence of arrested development. It has been pointed out that the facial part of the skull of Eric is so short that the frontal index is extremely high—67 instead of 54. Even more remarkable than the shortening of the face is the reduction in Eric of the capacity of the nasal chambers. In new-born foals, owing to the relatively large size of the cranium, the face is always more or less dished (fig. 7). In the case of the wild horse of Mongolia, the increase in the size of the nasal chambers soon gets rid of the dishing, and in course of time the nasal bones are bulged outwards, so as to give rise to a more or less marked “Roman-nose” (fig. 8). But in Eric and many other Shelties of the riding or Celtic type, owing to the expansion of the nasal chambers being prematurely arrested, the profile in the adult (fig. 6) differs but little from that of the new-born foal (fig. 7).
Shetland Ponies of the Cart-horse or “Forest” Type.—For want of material nothing very definite can be said about the nature of the dwarfing of ponies of the heavy or cart-horse type. In a typical 12-hands Celtic pony the metacarpals are 200 mm. long, 27·5 mm. wide, but in a typical 12-hands “forest” horse the metacarpals are, on an average, 193 mm. long and 35·1 mm. wide. An undwarfed, thick-set, 36-inch Sheltie built on the same lines as a 12-hands “forest” horse should have metacarpals about 145 mm. in length and 26·4 mm. in width, and should measure 5·5 inches below the knee.
The measurements available indicate that in a pedigree 36-inch Sheltie of the “forest” type, the front cannon-bones will probably measure 137 mm. by 26·2 mm., that the circumference below the knee will be 5·5 inches, and the distance from the elbow to the ground 21·5 inches—in Eric (36·5 inches) the length from elbow to the ground was 22·25 inches. If these figures are approximately correct, it follows that in a 36-inch Sheltie of the cart-horse type the limbs may be at least an inch shorter than in a dwarfed 36-inch pony of the riding type, and that the dwarfing may be unaccompanied by any loss of “bone.”
This conclusion is supported by the measurements of Odin, a 38-inch pony, 6 inches below the knee; of Vulcan, a 32-inch pony, 5 inches below the knee; and of other ponies of the Londonderry type belonging to the Ladies Hope, and also by those of Everlasting, Frederick, and other thick-set Auchlochan ponies. For example, in Everlasting, a 38-inch pony, the distance from the elbow to the ground is 22·75 inches, the circumference below the knee 5·5 inches, the bone being “round,” and the shank ·5 inches broader than in the flat-boned riding pony Eric.
In heavy horses, but especially in Shire colts, one (or more) of the limbs has occasionally an extra digit ending in a well-formed hoof. In a three-weeks’ horse embryo there are no rudiments of limbs; at four weeks the limbs are represented by paddle-like structures; at five weeks the paddles contain rudiments of three toes—miniatures of the three toes of Hypohippus (fig. 20).
In ordinary circumstances the development of the outer and inner toes (ii. and iv.) is soon arrested, but occasionally one of these rudiments develops into a toe as large and complete as in three-toed Miocene horses of the “forest” type. When this happens, when in addition to the third toe there is a toe corresponding to the human forefinger, we have a marvellous instance of reversion.
If in the Shetland breed there is a tendency to reversion, one would expect to find now and then extra digits in ponies of the heavy or “forest” type. I have, however, never heard of a Shetland pony with extra digits.
Dwarfing of the face and the reduction of the nasal chambers has apparently been carried further in some of the miniature cart-horses than in Eric and other flat-boned Shelties. In Jupiter,[F5] e.g., the head, though wider across the orbits, is shorter than in Eric, and decidedly more dished. Ancient horses adapted for a forest life had face pits in front of the orbits, which probably, like the corresponding pits in deer, lodged scent glands. Further, in ancient “forest” horses the upper lip was probably decidedly longer and more prehensible than in modern breeds. In broad-browed, big-boned Shetland ponies there is no indication of a pit for a scent gland, but there is sometimes an unusually long and decidedly mobile upper lip, which may or may not be due to reversion.
[F5] Jupiter was a 37·5 inch “elk-nosed” pony, with a girth of 54·5 inches, rounded quarters, a low set-on tail, a complete set of chestnuts, wide open hoofs, and six inches of “bone.”
The Causes of the Dwarfing of Shetland Ponies.—Given sufficient food and shelter, horses up to 15 hands—the size of the tallest prehistoric Old World wild horse—can easily be bred and reared in both the Western and Northern Islands of Scotland. On the other hand, large breeds are soon dwarfed when, in addition to a limited supply of food, the conditions during a considerable part of the year are extremely unfavourable. If Shetland ponies have not sprung from a small wild pigmy race, it may be safely asserted that their small size is mainly due to isolation in small areas where they were forced to shift for themselves under, as a rule, extremely unfavourable conditions.[F6] Obviously the environment may play a double part. It may (1) arrest growth by failing to provide sufficient food and shelter, and (2) it may eliminate the individuals which, by growing beyond a certain size, require during times of stress more food and shelter than are available. Considerable stress has been laid by writers on the dwarfing influence of the surroundings. It is said, e.g., that “horses taken to the barren and cold islands of Shetland become gradually smaller and hardier, like ponies, and the hair becomes thicker and longer. Long-continued exposure to such conditions ultimately results in the production of an animal like the Shetland pony, small in size, extremely hardy, able to withstand the most severe winter climate, and to subsist on a minimum of food.”[F7] It might be said that this view is supported by experiments in the Western Islands. At the beginning of the eighteenth century the Captain of Clanranald (who was killed in 1715 at the battle of Sheriffmuir) brought from Spain “some Spanish horses which he settled in his principal island of South Uist. These in a considerable degree altered and improved the horses in that and the adjacent islands. Even in 1764, not only the form but the cool fearless temper of the Spanish horse could be discerned in the horses of that island.... These at that time, both in figure and disposition, were thought the best horses observed in the Highlands, and though of low stature were judged more valuable than any other horses of the same size.” The descendants of the Spanish horses introduced by Clanranald for a time increased the size of the horses in the adjacent islands. Nevertheless, in spite of the introduction of the Spanish horses at the beginning of the eighteenth century, and the introduction of many south-country horses during the second half of the eighteenth century, the Highland and Island horses at the beginning of the nineteenth century were “sometimes only 9 and seldom 12 hands high.” Moreover, though some of them were “of an excellent form,” with “great strength in proportion to their size,” agile and spirited, many were “short necked, chubby headed, and thick and flat at the withers.”[F8]