The Origin of Species

wild.  So it is with young pheasants reared in England under a hen.  It is
not that chickens have lost all fear, but fear only of dogs and cats, for
if the hen gives the danger chuckle they will run (more especially young
turkeys) from under her and conceal themselves in the surrounding grass or
thickets; and this is evidently done for the instinctive purpose of
allowing, as we see in wild ground-birds, their mother to fly away.  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 under domestication 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 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 habit and selection
have probably concurred.

SPECIAL INSTINCTS.

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, namely, the instinct which leads the cuckoo to lay her eggs in
other birds' nests; the slave-making instinct of certain ants; and the
cell-making power of the hive-bee:  these two latter instincts have
generally and justly been ranked by naturalists as the most wonderful of
all known instincts.

INSTINCTS OF THE CUCKOO.

It is supposed by some naturalists that the more immediate cause of the
instinct of the cuckoo 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.  If this were the case the process of laying and hatching
might be inconveniently long, more especially as she migrates at a very
early period; and the first hatched young would probably have to be fed by
the male alone.  But the American cuckoo is in this predicament, for she
makes her own nest and has eggs and young successively hatched, all at the
same time.  It has been both asserted and denied that the American cuckoo
occasionally lays her eggs in other birds' nests; but I have lately heard
from Dr. Merrill, of Iowa, that he once found in Illinois a young cuckoo,
together with a young jay in the nest of a blue jay (Garrulus cristatus);
and as both were nearly full feathered, there could be no mistake in their
identification.  I could also give several instances of various birds which
have been known occasionally to lay their eggs in other birds' nests.  Now
let us suppose that the ancient progenitor of our European cuckoo had the
habits of the American cuckoo, and that she occasionally laid an egg in
another bird's nest.  If the old bird profited by this occasional habit
through being enabled to emigrate earlier or through any other cause; or if
the young were made more vigorous by advantage being taken of the mistaken
instinct of another species than when reared by their own mother,
encumbered as she could hardly fail to be by having eggs and young of
different ages at the same time, then the old birds or the fostered young
would gain an advantage.  And analogy would lead us to believe that the
young thus reared would be apt to follow by inheritance the occasional and
aberrant habit of their mother, and in their turn would be apt to lay their
eggs in other birds' nests, and thus be more successful in rearing their
young.  By a continued process of this nature, I believe that the strange
instinct of our cuckoo has been generated.  It has, also recently been
ascertained on sufficient evidence, by Adolf Muller, that the cuckoo
occasionally lays her eggs on the bare ground, sits on them and feeds her
young.  This rare event is probably a case of reversion to the long-lost,
aboriginal instinct of nidification.

It has been objected that I have not noticed other related instincts and
adaptations of structure in the cuckoo, which are spoken of as necessarily
co-ordinated.  But in all cases, speculation on an instinct known to us
only in a single species, is useless, for we have hitherto had no facts to
guide us.  Until recently the instincts of the European and of the non-
parasitic American cuckoo alone were known; now, owing to Mr. Ramsay's
observations, we have learned something about three Australian species,
which lay their eggs in other birds' nests.  The chief points to be
referred to are three:  first, that the common cuckoo, with rare
exceptions, lays only one egg in a nest, so that the large and voracious
young bird receives ample food.  Secondly, that the eggs are remarkably
small, not exceeding those of the skylark--a bird about one-fourth as large
as the cuckoo.  That the small size of the egg is a real case of adaptation
we may infer from the fact of the mon-parasitic American cuckoo laying
full-sized eggs.  Thirdly, that the young cuckoo, soon after birth, has the
instinct, the strength and a properly shaped back for ejecting its foster-
brothers, which then perish from cold and hunger.  This has been boldly
called a beneficent arrangement, in order that the young cuckoo may get
sufficient food, and that its foster-brothers may perish before they had
acquired much feeling!

Turning now to the Australian species:  though these birds generally lay
only one egg in a nest, it is not rare to find two and even three eggs in
the same nest.  In the bronze cuckoo the eggs vary greatly in size, from
eight to ten lines in length.  Now, if it had been of an advantage to this
species to have laid eggs even smaller than those now laid, so as to have
deceived certain foster-parents, or, as is more probable, to have been
hatched within a shorter period (for it is asserted that there is a
relation between the size of eggs and the period of their incubation), then
there is no difficulty in believing that a race or species might have been
formed which would have laid smaller and smaller eggs; for these would have
been more safely hatched and reared.  Mr. Ramsay remarks that two of the