But this loose employment of the term "instinct" really accords with the
nature of the thing; for it is wholly impossible to draw any line of
demarcation between reflex actions and instincts. If a frog, on the
flank of which a little drop of acid has been placed, rubs it off with
the foot of the same side; and, if that foot be held, performs the same
operation, at the cost of much effort, with the other foot, it certainly
displays a curious instinct. But it is no less true that the whole
operation is a reflex operation of the spinal cord, which can be
performed quite as well when the brain is destroyed; and between which
and simple reflex actions there is a complete series of gradations. In
like manner, when an infant takes the breast, it is impossible to say
whether the action should be rather termed instinctive or reflex.
What are usually called the instincts of animals are, however, acts of
such a nature that, if they were performed by men, they would involve
the generation of a series of ideas and of inferences from them; and it
is a curious, and apparently an insoluble, problem whether they are, or
are not, accompanied by cerebral changes of the same nature as those
which give rise to ideas and inferences in ourselves. When a chicken
picks up a grain, for example, are there, firstly, certain sensations,
accompanied by the feeling of relation between the grain and its own
body; secondly, a desire of the grain; thirdly, a volition to seize it?
Or, are only the sensational terms of the series actually represented in
consciousness?
The latter seems the more probable opinion, though it must be admitted
that the other alternative is possible.
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