[NOTE.-Herbert Spencer, who accepts Darwin's inference, explains how the fear of man, acquired by experience, becomes instinctive in birds, in the following passage: "It is well known that in newly-discovered lands not inhabited by man, birds are so devoid of fear as to allow themselves to be knocked over with sticks; but that, in the course of generations, they acquire such a dread of man as to fly on his approach: and that this dread is manifested by young as well as by old. Now unless this change be ascribed to the killing-off of the least fearful, and the preservation and multiplication of the most fearful which, considering the comparatively small number killed by man, is an inadequate cause, it must be ascribed to accumulated experience; and each experience must be held to have a share in producing it. We must conclude that in each bird that escapes with injuries inflicted by man, or is alarmed by the outcries of other members of the flock (gregarious creatures of any intelligence being necessarily more or less sympathetic), there is established an association of ideas between the human aspect and the pains, direct and in-direct, suffered from human agency. And we must further con-clude, that the state of consciousness which compels the bird to take flight, is at first nothing more than an ideal reproduction of those painful impressions which before followed man's approach; that such ideal reproduction becomes more vivid and more massive as the painful experiences, direct or sympathetic, increase; and that thus the emotion, in its incipient state, is nothing else than an aggregation of the revived pains before experience. "As, in the course of generations, the young birds of this race begin to display a fear of man before yet they have been injured by him, it is an unavoidable inference that the nervous system of the race has been organically modified by these experiences, we have no choice but to conclude, that when a young bird is led to fly, it is because the impression produced in its senses by the approaching man entails, through an incipiently reflex action, a partial excitement of all those nerves which in its ancestors had been excited under the like conditions; that this partial excitement has its accompanying painful consciousness, and that the vague painful consciousness thus arising constitutes emotion proper--_emotion undecomposable into specific experiences, and, therefore, seemingly homogeneous"_ (Essays, vol. i. p. 320.)] It is comforting to know that the "unavoidable inference" is, after all, erroneous, and that the nervous system in birds has not yet been organically altered as a result of man's persecution; for in that case it would take long to undo the mischief, and we should be indeed far from that "better friendship" with the children of the air which many of us would like to see. CHAPTER VI. PARENTAL AND EARLY INSTINCTS.