SECRETS OF THE HORSE

Otago Witness, Issue 4031, 16 June 1931, Page 10

SECRETS OF THE HORSE

By Professor Sir J. Arthur Thomson. A highly evolved creature like the horse, in many ways the ablest of those mammals which man has taken into partnershipwith him, has many secrets still imperfectly understood. Let us select three or four of them. Julius Casar used to ride on a two-toed horse, which no one else was allowed to use. It was for a time commemorated in a statue in Rome—a horse with a cloven hoof! Even in spite of the statue and. the historians, one might have been tempted to doubt the reality of this quaint steed, but there are several records of similar aberrations—sometimes with three toes. What is the meaning nf the occurrence of these rare polydactylous horses? ¥ ¥• It must be understood that the modern horse walks and runs on the middle <>r third digit of each hand or foot; and it is amazing to think of a horse clearing a distance of eight feet and alighting safely with all its weight on the tip of the finger 111 of the two hands—a marvel of adaptation! The third digit, with three joints as usual, is carried on the end of the third meta podial (metacarpal for the fore-limb, metatarsal for the hind-limb) which corresponds to the middle bone in the palm of our hand or in the instep of our foot.

Now, in'each side of this strong vertical palm bone or instep bone, there is a slender splint, and these are vestiges of the metapodials II and IV. But at the lower end of each splint there is a little bony swelling called the “ button.” These are separate in the embryo, and they have been shown by Professor Cossar Ewart to be vestiges of the missing joints of fingers or toes II and IV. Thus the past of the horse lives on in its present, for the earliest fossil horse, Eohippus from the London clay (living over two million years ago), had four fingers and throe toes. Here wc surprise, so to speak, one of the secrets of the horse, for Cresar’s steed may have been due to a rehabilation or re-activa-tion of hereditary items which normally lie in latency. Or, on another theory, a modern two-toed or three-toed horse may be, apart from distant ancestry altogether, a new departure or mutation in the line of doubling an existing part, just as in six-fingered children. Tn an interesting paper recently published on the “ Genetics of the Horse,” Professors F. A. E. Crew and Ifr A. D. Buchanan Smith have brought together an array of facts bearing on the kind of problem we are dscussing. Thus they ask how is it that little horses or ponies often occur on islands? It there some direct connection between restricted space and small size, as was suggested in one of the few jokes made by Herbert Spencer, who characterised a chop be once enjoyed in the Isle of Wight as being very large for so small an island!

Water snails can be kept from growing big if they are reared in a shape of vessel that does not allow them sufficient surface for exercise; does something like this apply to island ponies? Darwin was evidently inclined to this line of interpretation, for he says: “There can lie no doubt that horses become greatly reduced in size and altered in appearance by living on mountains and islands; and this apparently is due to want of nutritious or varied food.” When the reduction in size is known to have come about in a few generations, as is alleged in the case of the horses of the Falkland Islands, the theory must be entertained that severe conditions, both nutritive and environmental, acted prejudicially on the mares in foal, and subsequently on the foals themselves, the result being individuals of stunted growth and some enfeeblement. But island ponies, like “ Shelties,” show no lack of vigour.

Another theory would be possible in cases where the change was known to have gone on through many generations, for in such cases there might be a gradual elimination of constitutions tending to bigness and a relatively greater survival of variants in the direction of resistant dwarfs. Thus a racial change might be effected. The interpretation remains a secret, but a very striking instance of the fact is given by Crew and Smith. It concerns the inhospitable Sable Island off Nova Scotia, where horses were twice marooned, in 1760 and at an earlier date. When Gilpin (not the famous John) visited the island about 1864 there were 400 wild ponies in about six herds. “ The animals were quite wild, each herd had its own feeding ground, the males were* very fierce and appeared to sleep standing. Thus, in about 150 years the horses had returned to the habits of the wild tarpan horse, which they resembled in size, hairy heads, and thick coats.” It should be remembered that the earliest extinct horses were not bigger than fox terriers, and in the light of this ancestral smallness, a somewhat sudden reversion from horses to ponies becomes more readily intelligible. A mule in the stricter sense is the result of crossing a pony mare with a male donkey or “ jack.” The hybrid may be, of course, a male or a female. No one seems to know much about the “ hinny ”•—the result of crossing a female

ass (a “jenny” or “jennet”) with a stallion pony. Keeping to mules in the stricter sense, we may note the conclusion reached by Crew and Smith after sifting the recorded cases: “ Recent evidence shows that, however rarely it may happen that a mare mule becomes a parent, this undoubtedly does take place occasionally.” This seems also tone true of a mare hinny. But the chances of fertility in the male mule are practically negligible. Yet there is a case reported from the Texas College of Agriculture, where the foal of a mare mule by a stallion proved himself the si ire of at least five foals. One might think it easy to make sure of these things, but there are strange snags. Thus unmated female mules occasionally give milk, and some of the alleged “ sure and certain ” cases of fertility are instances of such females adopting and suckling a pony foal!

In every cell of our body, except the

fully-formed red blood corpuscles, there is a nucleus, and this contains a definite number of readily stainable bodies, often like rodlets, called chromosomes. Man’s number is 48. As the same number

occurs in some other organisms, e.g., in one of the snails and in one of the

plaintans, it is not the number in itself that is very important, but its constancy throughout the cells of the body. Now, the horse’s number is 60, and one of the reasons why mules are not readily fertile

may be that the donkey’s number is different —perhaps 64. This is another secret. —John o’ London’s Weekly.