Instinct aims at certain definite ends; but these ends cannot always be reached by the same means, especially when places and circumstances are not the same. Accommodation is necessary, or it could not always produce the effects for which it is intended. Would the instinct of the spider be complete, if, after it has guided her to spin a web so neat and trim and regular, it did not also lead her to repair her broken snare, when the cords have been sundered by the struggles of some powerful captive? But this pliancy of the spider's instinct is no more remarkable than the contingent operation of the instincts of many species of animals. "It is remarkable," says Kirby, "that many of the insects which are occasionally observed to emigrate are not usually social animals, but seem to congregate, like swallows, merely for the purpose of emigration." When certain rare emergencies occur, which render it necessary for the insects to migrate, a contingent instinct develops itself, and renders an unsocial species gregarious. It is probable that most of our domesticated species, exhibiting as they do in that condition attainments foreign to their natural habits and faculties in a wild state, were endowed with provisional instincts with a view to their association with man. But generally the docility of animals does not extend to attainments which are radically different from their habits and faculties in a wild state. Casual acquirements, which have no relation to their exigencies in their natural condition, never become hereditary, and are not, therefore, instinctive. A young pointer-dog, which has never been in the fields before, will not only point at a covey of partridges, but will remain motionless, like a well-trained dog. The fact that the sagacity of the pointer is hereditary shows that it is the development of an instinctive propensity; for simple knowledge is not transmitted by blood from one generation to another. We have heard of a pig that pointed game, and of another that was learned in letters; but we ascertain in every such instance that their foreign acquirements do not reappear in their progeny, but end with the pupils of the time being. The pig's peculiarity of pointing did not arise from the development of a provisional instinct, because it does not become hereditary; but the same act in the pointer-dog is instinctive,--for, when once brought out by associating with man, it has remained with the breed, being a part of the animal's nature, which existed in embryo till it was developed by a companionship with man, for whose use this faculty was alone intended.