VIII. On the Hereditary Diseases of Horses. By FINLAY DUN, Jun., V.S., Lecturer on Materia Medica, &c., at the Edinburgh Veterinary College. PRIZE ESSAY. OUR everyday experience of the production and development of plants and animals at once suggests the existence of the great natural law embodied in the familiar saying, “like produces like." In accordance with this law the peculiar properties, characters, and qualities of the parent-whether good or bad, healthy or diseased, external or internal-are transmitted to the offspring, or, in a word, are hereditary. To illustrate this natural law of hereditary transmission, with especial reference to the diseases of horses and cattle, is the object of this report, and, in treating of the subject, we shall notice I. General hereditary characters, both healthy and diseased. II. The hereditary diseases of horses. III. The hereditary diseases of cattle. I. Many interesting and valuable facts have been recorded which prove, beyond all doubt, the hereditary tendency of many of the physical, mental, and moral qualities of man. Parents transmit to their children their own-or, at all events, similar-external forms, similar intellectual capacities, temperaments, dispositions, virtues, and vices, as well as similar tendencies to particular diseases. Certain families are remarkable, during many centuries, for tall and handsome figures, and for a striking similarity of features; whilst others perpetuate a less perfect form, the peculiar deformities of the parents reappearing in the children of each successive generation. For example, the thick upper lip of the members of the imperial house of Austria has been a characteristic of the family for centuries; and every one is familiar with the curious case of the Yorkshire family with their six fingers and toes, which remarkable conformation has continued for several generations; and other analogous cases are recorded.* But the hereditary transmission of external form is exemplified, on a more extended scale, by the striking resemblance often observed amongst the different individuals of a community or race, even where these are exposed to different external agencies. The cases of the Jews and the Gipsies will suggest 'Researches into the Physical History of Mankind,' by James C. Prichard, 3rd edition, pp. 244-5. See also, at pp. 347-9, the description of a man whose skin was greatly thickened and covered with warty excrescences, and in whose descendants these peculiarities were noticeable in the third generation. themselves to every one as most apposite examples. Although exposed for centuries to the powerfully modifying influences of external circumstances of climate, country, association with nations of very different customs and habits, these remarkable races still retain their identity, and remain distinct and peculiar people. But it is not alone their face or figure that remains unaltered, their manners, habits, and customs are also uniform and permanent: a most striking proof of the hereditary transmission of almost every bodily and mental character and quality. As regards intellectual ability, it is observed that certain races are remarkable for intelligence and aptitude in the acquirement of knowledge, and others for stupidity and narrowness of capacity; that the children of such races, although reared and educated with equal care, always show much difference in intellectual attainments; and that it is only after educating several generations of the less-gifted race that they attain the natural capacity of the more gifted. Both ancient and modern history afford many striking instances of analogous temperaments and dispositions being transmitted from father to son through many generations; of some families remarkable during centuries for virtue, honour, and liberality, and of others notorious during an equally long period for every sort of wickedness, vice, and oppression. But diseases, as well as physical and mental qualities, descend from parent to children. Many of the most wide-spread and fatal maladies affecting the human subject are hereditary. Under this category we may include pulmonary consumption, which destroys so many of the inhabitants of these islands, frequently decimating, and sometimes completely sweeping away, entire families; scrofula, gout, gravel, and rheumatism, which, like consumption, occur chiefly in predisposed subjects, and in the progeny of those who have themselves suffered from them; most nervous diseases, especially palsy, epilepsy, and insanity, which rarely attack any individual without also affecting many of the same family; and many imperfections of the external senses, as deafness and blindness. These are the most common hereditary diseases incident to man; most of them have their analogues in the lower animals, in which they are also hereditary. Amongst horses and cattle we find, as in the human subject, ample illustration of the hereditary tendency of external form, disposition, habit, and disease. The parent transfers to its offspring size, shape, and general conformation similar to its own; and the aphorism "like produces like" is as applicable to faulty and disproportioned as to beautiful and symmetrical form, to diseased and debilitated as to healthy and vigorous constitution, to gentle and tractable as to fiery and indomitable disposition. The size, weight, general appearance, expression of countenance, fleetness, and temper of the horse are all hereditary. Many illustrations might be given of particular families being remarkable during several generations for good or bad points, as for well or ill-formed head; for high and well-developed, or for low and weak withers; for fine, strong, and well-turned, or for coarse, weak, and ill-formed limbs. Peculiarities of colour often extend through many generations, and are so constant in their transmission as sometimes to form one of the distinctive characteristics of a race. Indeed, most breeds of horses have a prevailing colour, to which there are few exceptions. The heavy horses of Lincolnshire, for example, are generally black; the Cleveland, bay; and the wild horses of the plains of Eastern Siberia, dun. Particular markings, also -as white spots on various parts of the body, stars and blazes on the face, one or more white feet or legs-often continue for many generations peculiar to certain families. The general constitution of an animal is no less hereditary than the external qualities to which we have just alluded. Some stocks of horses, for example, can sustain with impunity an amount of labour which, in others of the same breed, would cause serious bad effects; and the peculiar action both of medicines, and of morbific causes, is generally observed to be similar in members of the same family. But besides the general constitution of the parents, their special condition at the time of copulation also appears to be to a certain extent transmitted to the offspring; and hence the necessity of selecting for breeding purposes only animals of a strong and healthy constitution, and of using them only when they are in full possession of all their physical energies. For a high state of the physical energies at the time of impregnation is believed to induce a correspondingly great development of physical power in the offspring; and of this we have a curious example in the fact, that the Arabs, before bringing the parents together, give them a short gallop, believing that the spirit and fleetness of the progeny is thereby enhanced. On the other hand, we find that even a slight and temporary debility at the time of copulation exercises a marked deteriorating effect upon the spirit and vigour of the offspring, and it is well known that the stock of old stallions is generally weak and spiritless: "Senes valetudinarii, imbecilles . . . filios vitiosa consti tutione gignunt.”—Fernel. It must be observed that external circumstances, as diet and temperature, exercise a powerful influence on animal growth and development. With meagre fare and exposure to cold, animals do not reach the average size of their race, and beget stock as much below average as themselves. In similarly unfavourable circumstances, these again do not reach the size even of their own immediate parents, and procreate a still smaller progeny. Conditions favourable to growth and improvement operate in a similar manner. They improve each individual, and the descendants of each inherit to a greater or less degree the improvements on the parent stock. Animals, then, are altered by circumstances, and transmit to their progeny their altered forms. Thus, after a few generations, the external characters of a breed are often greatly modified, and hence have arisen the permanent varieties of horses and cattle met with in different parts of the kingdom-the tall heavy horse of the Lincolnshire fens, the light, active, but powerful thorough-bred, the small pony of Shetland and amongst cattle, the short-horned, the Ayrshire, and West Highland breeds, and many others-varieties which have a common origin, but which are now so distinct and permanent that each produces a progeny with its own distinctive characteristics. Thus, even acquired and artificial habits may become hereditary. Certain districts are famous for their trotting horses, and many Irish hunters are remarkable for their peculiar style of leaping. Some years ago the Earls of Morton and Zetland imported from Dongolia, in Upper Egypt, several entire horses, which were remarkable for their high and prancing action. Their progeny, both out of thorough-bred mares and those of the heavier breeds, inherited the action of the sires to such a degree that they had all to be sold as carriage-horses, being unfit for racing, hunting, or almost any other kind of work. Prichard states, in his Natural History of Man,'* that the horses bred on the table-lands of the Cordilleras "are carefully taught a peculiar pace, which is a sort of running amble;" after a few generations this pace becomes a natural one, young untrained horses adopting it without compulsion. But what is still more curious is the fact, that, if these domesticated stallions breed with mares of the wild herds which abound in the surrounding plains, they "become the sires of a race to which the ambling pace is natural and requires no teaching." "The hereditary propensities of the offspring of Norwegian ponies," says Mr. T. A. Knight, in a paper read before the Royal Society in 1837, "whether full or half bred, are very singular. Their ancestors have been in the habit of obeying the voice of their riders, and not the bridle, and the horsebreakers complain that it is impossible to produce this last habit in the young colts; they are notwithstanding exceedingly docile and obedient when they understand the commands of their master. It is equally difficult to keep them within hedges, owing, per *Second Edition, p. 35. haps, to the unrestrained liberty to which the race may have been accustomed in Norway."* Much of what has been already stated concerning the hereditary nature of the external conformation and other qualities of the horse is also applicable to cattle. The progeny of a common stock bear a close resemblance to their parents and to each other in general appearance, length of limb, development of chest, shape of carcase, position and size of the udder, adaptation for the dairy, thickness of skin, and length and texture of the hair. In some of the hot provinces of South America there are cattle "noted for an extremely rare and fine fur. . . . The variety is reproduced or descends in the stock."† In the same localities is also found another race with an entirely naked skin, which peculiarity is also hereditary. In our own country, too, there are great differences in the length and texture of the hair of various sorts of cattle-differences which, as in the South American animals, are transmitted to the progeny. The existence or non-existence of,horns, their size, shape, and curvatures, are characters the hereditary nature of which is generally admitted. But defects and deformities may also become permanent in a stock. We are informed by a friend that he has seen several cattle with a small portion of skin covered with short hair situated on the eye, just within its outer canthus ; and that this peculiarity had been traced back for five or six generations, and had occurred in every case in exactly the same spot of the right eye. We have deemed it advisable thus far to consider the hereditary tendencies of external form, of habit, and of constitution, in order to illustrate more fully and satisfactorily the hereditary tendencies of disease, which we shall now proceed to discuss. Hereditary diseases exhibit certain eminently characteristic phenomena, some of which we shall here enumerate : 1. They are transmitted by the male as well as by the female parent, and are doubly severe in the offspring of parents both of which have been affected by them. 2. They develop themselves, not only in the immediate progeny of animals affected by them, but also in many subsequent generations. 3. They do not, however, always appear in each generation exactly in the same form. One disease is sometimes substituted for another analogous to it, and this, after some generations, becomes again changed into that to which the breed was originally liable. Thus, stocks of cattle previously subject to phthisis often become affected for several generations with *See Prichard's Natural History of Man,' 2nd edition, p. 72. |