edicament, 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 referr