But there are cases where a knowledge and careful examination of the form and size of the mirror becomes of the greatest importance. It is well known that certain signs or marks of great milkers are developed only as the capacities of the animal herself are fully and completely developed by age. The milk-veins, for instance, are never so large and prominent in heifers and young cows as in old ones, and the same may be said of the udder, and the veins of the udder and perineum; all of which it is of great importance to observe in the selection of milch cows. Those signs, then, which in cows arrived at maturity are almost sufficient in themselves to warrant a conclusion as to their merits as milkers, are, to a great extent, wanting in younger animals, and altogether in calves, of which there is often doubt whether they shall be raised; and here a knowledge of the form of the mirror is of immense advantage, since it gives, at the outset, and before any expense is incurred, a somewhat reliable means of judging of the future milking capacities of the animal or, if a male, of the probability of his transmitting milking qualities to his offspring. It will be seen, from an examination of the points of a good milch cow, that, though the same marks which indicate the greatest milking qualities may not indicate any great aptitude to fatten, yet that the signs which indicate good fattening qualities are included among the signs favorable to the production of milk, such as soundness of constitution, indicated by good organs of digestion and respiration, fineness and mellowness of the skin and hair, quietness of disposition, which inclines the animal to rest and lie down in chewing the cud, and other marks which are relied on by graziers in selecting animals to fatten. In buying dairy stock the farmer generally finds it for his interest to select young heifers. They give the promise of longer usefulness. But it is often the case that older cows are selected with the design of using them for the dairy for a limited period, and then feeding them for the butcher. In either case, it is advisable, as a rule, to choose animals in low or medium condition. The farmer cannot ordinarily afford to buy fat; it is more properly his business to make it, and to have it to sell. Good and well-marked cows in poor condition will rapidly gain in flesh and products when removed to better pastures and higher keeping, and they cost less in the original purchase.