But this instinct retained by
our chickens has become useless under domestication, for the
mother-hen has almost lost by disuse the power of flight.
Hence, we may conclude, that domestic instincts have been acquired and
natural instincts have been lost partly by habit, and partly by man
selecting and accumulating during successive generations, peculiar
mental habits and actions, which at first appeared from what we must
in our ignorance call an accident. In some cases compulsory habit
alone has sufficed to produce such inherited mental changes; in other
cases compulsory habit has done nothing, and all has been the result
of selection, pursued both methodically and unconsciously; but in most
cases, probably, habit and selection have acted together.
We shall, perhaps, best understand how instincts in a state of nature
have become modified by selection, by considering a few cases. I will
select only three, out of the several which I shall have to discuss in
my future work,--namely, the instinct which leads the cuckoo to lay
her eggs in other birds' nests; the slave-making instinct of certain
ants; and the comb-making power of the hive-bee: these two latter
instincts have generally, and most justly, been ranked by naturalists
as the most wonderful of all known instincts.
It is now commonly admitted that the more immediate and final cause of
the cuckoo's instinct is, that she lays her eggs, not daily, but at
intervals of two or three days; so that, if she were to make her own
nest and sit on her own eggs, those first laid would have to be left
for some time unincubated, or there would be eggs and young birds of
different ages in the same nest.
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