n 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 {217} 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 has to migrate 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 asserted that the American cuckoo occasionally
lays her eggs in other birds' nests; but I hear on the high authority of
Dr. Brewer, that this is a mistake. Nevertheless, I could 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; but that
occasionally she laid an egg in another bird's nest. If the old bird
profited by this occasional habit, or if the young were made more vigorous
by advantage having been taken of the mistaken maternal instinct of another
bird, than by their own mother's care, encumbered as she can 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 me 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
successful in rearing their young. By a continued process of this nature, I
believe that the strange instinct of our cuckoo could be, and has been,
{218} generated. I may add that, according to Dr. Gray and to some other
observers, the European cuckoo has not utterly lost all maternal love and
care for her own offspring.

The occasional habit of birds laying their eggs in othe