The knowledge of this law[13] gives us a clue to the cause of many of the disappointments of which practical breeders often complain and to the cause of many variations otherwise unaccountable, and it suggests particular caution as to the first male employed in the coupling of animals, a matter which has often been deemed of little consequence in regard to cattle, inasmuch as fewer heifers' first calves are reared, than of such as are borne subsequently.
Another faint ray of light touching the causes of variation is afforded us by the fact that the qualities of offspring are not only dependent on the habitual conditions of the parents, but also upon any peculiar condition existing at the time of sexual congress. For instance, the offspring of parents ordinarily healthy and temperate, but begotten in a fit of intoxication, would be likely to suffer permanently, both physically and mentally, from the condition which the parents had temporarily brought upon themselves. On the other hand, offspring begotten of parents in an unusually healthy and active condition of body and mind, would likely be unusually endowed both mentally and physically. The Arabs in breeding horses take advantage of this fact, for before intercourse, both sire and dam are actively exercised, not to weariness, but sufficiently to induce the most vigorous condition possible. Of this, too, we have proof in the phenomenon sometimes observed by breeders, that a strong mental impression made upon the female by a particular male, will give the offspring a resemblance to him, even though she have no sexual intercourse with him. Of this, Mr. Boswell in his prize essay published in 1828, gives a remarkable instance. He says that Mr. Mustard of Angus, one of the most intelligent breeders he had ever met with, told him that one of his cows chanced to come into season while pasturing on a field bounded by that of one of his neighbors, out of which field an ox jumped and went with the cow until she was brought home to the bull. The ox was white, with black spots, and horned. Mr. Mustard had not a horned beast in his possession, nor one with any white on it. Nevertheless, the produce of the following spring was a black and white calf with horns.