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over-sensitive; the touch of any object there drives him frantic, and
if it is continued, it releases the impulse to walk upwards and back-
wards until he has reached the edge of whatever he is walking on, and
then to tilt the object overboard. He will behave in just the same
way to marbles or hazel-nuts or any other small object. Indeed, if
you think of it, he cannot know what lie is doing. For he will act thus
immediately he is hatched, before his eyes are open ; even if he could
be taught, his parents have never been near him, and his foster-
parents are hardly likely to instruct him in this particular! No, the
whole train of actions is the outcome of a marvellous piece of machin-
ery with which he is endowed by heredity, just as he is endowed with
the equally marvellous adaptive mechanism of his feathers. The
machinery consists in the shape of the back, its hyper-sensitiveness,
and the intricate pattern of nervous connections in the brain and
spinal cord which set the particular muscles into action. The act in
fact is purely instinctive, just as instinctive and automatic as sneezing
or coughing in ourselves. And, like coughing, it has been brought
into being by the long unconscious processes of natural selection, not
by any foresight or conscious will.
Once the foster-brothers arc outside, we shall get another surprising
peep into bird mind. When the foster-mother comes home, she docs
not seem in the least distressed by the absence of all but one of her
brood, but at once sets about feeding the changeling. What is more,
she pays no attention to her own offspring, even should some of these
be dangling just outside the nest. As long as there is something in the
nest which appeals to her parental instincts, it seems that young birds
outside the nest, even if they be her own, are treated as so many
foreign objects.
Then the young cuckoo begins to grow. It grows into a creature
entirely different from its foster-parents, and eventually becomes
several times bulkier than they, so that they have to perch on its head
to drop food into its mouth! But they are not in the least discon-
certed, as would human parents if their children began growing into
giants, and giants of quite a different appearance from themselves.
They are built to respond to the stimulus of appeals for food from any
nestling that starts life in their nest, and they continue their response,
whether the nestling is their own or a cuckoo.
At last the young cuckoo is ready to fly, leaves his foster-parents,
and very soon must leave the country on migration. So far as we
know, all the old cuckoos have before this time left the country for the
south, so that it is again without any teaching or any knowledge that
the young ones must obey the migration urge*