then arises, how did this unnecessary fear, so universal in swallows,
originate? Can it be a survival of a far past--a time when some
wide-ranging small falcon, aerial in habits as the swallow itself,
preyed by preference on hirundines only ?

[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.