The Descent of Man

east against the wires of her cage, until it is
bare and bloody. It causes young salmon to
leap out of the fresh water, in which they could
continue to exist, and thus unintentionally to
commit suicide. Every one knows how strong
the maternal instinct is, leading even timid
birds to face great danger, though with hesitation,
and in opposition to the instinct of selfpreservation.
Nevertheless, the migratory instinct
is so powerful, that late in the autumn
swallows, house-martins, and swifts frequently
desert their tender young, leaving them to perish
miserably in their nests. (22. This fact, the
Rev. L. Jenyns states (see his edition of 'White's
Nat. Hist. of Selborne,' 1853, p. 204) was first
recorded by the illustrious Jenner, in 'Phil.
Transact.' 1824, and has since been confirmed
by several observers, especially by Mr. Blackwall.
This latter careful observer examined, late
in the autumn, during two years, thirty-six
nests; he found that twelve contained young
dead birds, five contained eggs on the point of