Judicious selection is indispensable to success in breeding, and this should have regard to every particular--general appearance, length of limb, shape of carcass, development of chest; in cattle, to the size, shape, and position of the udder, thickness of skin, touch, length and texture of hair, docility, and all those points which go to make up the desirable animal. Not only should care be exercised 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, oftentimes, no obvious peculiarity of structure or appearance which suggests the possession of diseases or defects which are transmissible; and for this reason, special care and continued acquaintance are requisite in order to be assured of their absence in breeding animals; but such a tendency, although invisible or inappreciable to careless observers, must still, judging from its effects, have as real and certain an existence as any peculiarity of form or color. In 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 afterward breaks out with all its former severity. The diseases which are found hereditary in cattle are scrofula, consumption, dysentery, diarrhoea, rheumatism, and malignant tumors. As these animals are less exposed to the exciting causes of disease, and less liable to be overtasked or subjected to violent changes of temperature, or otherwise put in jeopardy, their diseases are not so numerous as those of the horse, and what they have are less violent, and generally of a chronic character. With regard to hereditary diseases, it is eminently true that "an ounce of prevention is worth a pound of cure." As a general and almost invariable rule, animals possessing either defects or a tendency to disease, should not be employed for breeding. If, however, for special reasons it seems desirable to breed from one which has some slight defect of symmetry, or a faint tendency to disease--although for the latter it is doubtful whether the possession of any good qualities can fully compensate--it should be mated with one which excels in every respect in which it is itself deficient, and on no account with one which is near of kin to it. There is another law, by which that of similarity is greatly modified--the law of _Variation_ or divergence.