The Principles of Breeding | Page 7

shape of carcass, development of chest; if in cattle, the size, shape and position of udder, thickness of skin, "touch," length and texture of hair, docility, &c., &c.; if in horses, their adaptation to any special excellence depending on form, or temperament, or nervous energy.
Not only should care be taken to avoid structural defects, but especially to secure freedom from hereditary diseases, as both defects and diseases appear to be more easily transmissible than desirable qualities. There is often no obvious peculiarity of structure, or appearance, indicating the possession of diseases or defects which are transmissible, and so, special care and continued acquaintance are necessary in order to be assured of their absence in breeding animals; but such a tendency although invisible or inappreciable to cursory observation, must still, judging from its effects, have as real and certain an existence, as any peculiarity of form or color.
Every one who believes that a disease may be hereditary at all, must admit that certain individuals possess certain tendencies which render them especially liable to certain diseases, as consumption or scrofula; yet it is not easy to say precisely in what this predisposition consists. It seems probable, however, that it may be due either to some want of harmony between different organs, some faulty formation or combination of parts, or to some peculiar physical or chemical condition of the blood or tissues; and that this altered state, constituting the inherent congenital tendency to the disease, is duly transmitted from parent to offspring like any other quality more readily apparent to observation.
Hereditary diseases exhibit certain eminently characteristic phenomena, which a late writer[2] enumerates as follows:
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 are affected by them.
2. They develop themselves not only in the immediate progeny of one affected by them, but also in many subsequent generations.
3. They do not, however, always appear in each generation in the same form; one disease is sometimes substituted for another, analogous to it, and this again after some generations becomes changed into that to which the breed was originally liable--as phthisis (consumption) and dysentery. Thus, a stock of cattle previously subject to phthisis, sometimes become affected for several generations with dysentery to the exclusion of phthisis, but by and by, dysentery disappears to give place to phthisis.
4. Hereditary diseases occur to a certain extent independently of external circumstances; appearing under all sorts of management, and being little affected by changes of locality, separation from diseased stock, or such causes as modify the production of non-hereditary diseases.
5. They are, however, most certainly and speedily developed in circumstances inimical to general good health, and often occur at certain, so called, critical periods of life, when unusual demands on the vital powers take place.
6. They show a striking tendency to modify and absorb into themselves all extraneous diseases; for example, in an animal of consumptive constitution, pneumonia seldom runs its ordinary course, and when arrested, often passes into consumption.
7. Hereditary diseases are less effectually treated by ordinary remedies than other diseases. Thus, although an attack of phthisis, rheumatism or opthalmia may be subdued, and the patient put out of pain and danger, the tendency to the disease will still remain and be greatly aggravated by each attack.
In horses and neat cattle, hereditary diseases do not usually show themselves at birth, and sometimes the tendency remains latent for many years, perhaps through one or two generations and afterwards breaks out with all its former severity."
The diseases which are found to be hereditary in horses are scrofula, rheumatism, rickets, chronic cough, roaring, ophthalmia or inflammation of the eye,--grease or scratches, bone spavin, curb, &c. Indeed, Youatt says, "there is scarcely a malady to which the horse is subject, that is not hereditary. Contracted feet, curb, spavin, roaring, thick wind, blindness, notoriously descend from the sire or dam to the foal."
The diseases which are found hereditary in neat cattle are scrofula, consumption, dysentery, diarrhea, rheumatism and malignant tumors. Neat cattle being less exposed to the exciting causes of disease, and less liable to be overtasked or exposed to violent changes of temperature, or otherwise put in jeopardy, their diseases are not so numerous, and what they have are less violent than in the horse, and generally of a chronic character.
Scrofula is not uncommon among sheep, and it presents itself in various forms. Sometimes it is connected with consumption; sometimes it affects the viscera of the abdomen, and particularly the mesenteric glands in a manner similar to consumption in the lungs. The scrofulous taint has been known to be so strong as to affect the foetus, and lambs have occasionally been born with it, but much oftener they show it at an early age, and any affected