by the heat of incipient fermentation, would in the long
run be more freely hatched and would probably produce young ones with
the same more highly developed collecting tendencies; of these again,
those with the best developed powers would again tend to rear most
young. Thus this strange instinct might _possibly_ be acquired, every
individual bird being as ignorant of the laws of fermentation, and the
consequent development of heat, as we know they must be.

Secondly, to take the case of animals feigning death (as it is commonly
expressed) to escape danger. In the case of insects, a perfect series
can be shown, from some insects, which momentarily stand still, to
others which for a second slightly contract their legs, to others which
will remain immovably drawn together for a quarter of an hour, and may
be torn asunder or roasted at a slow fire, without evincing the smallest
sign of sensation. No one will doubt that the length of time, during
which each remains immovable, is well adapted to <favour the insect's>
escape <from> the dangers to which it is most exposed, and few will deny
the _possibility_ of the change from one degree to another, by the means
and at the rate already explained. Thinking it, however, wonderful
(though not impossible) that the attitude of death should have been
acquired by methods which imply no imitation, I compared several
species, when feigning, as is said, death, with others of the same
species really dead, and their attitudes were in no one case the same.

Thirdly, in considering many instincts it is useful to _endeavour_ to
separate the faculty{288} by which they perform it, and the mental power
which urges to the performance, which is more properly called an
instinct. We have an instinct to eat, we have jaws &c. to give us the
faculty to do so. These faculties are often unknown to us: bats, with
their eyes destroyed, can avoid strings suspended across a room, we know
not at present by what faculty they do this. Thus also, with migratory
birds, it is a wonderful instinct which urges them at certain times of
the year to direct their course in certain directions, but it is a
faculty by which they know the time and find their way. With respect to
time{289}, man without seeing the sun can judge to a certain extent of
the hour, as must those cattle which come down from the inland mountains
to feed on sea-weed left bare at the changing hour of low-water{290}. A
hawk (D'Orbigny) seems certainl