r birds' nests,
either of the same or of a distinct species, is not very uncommon with the
Gallinaceae; and this perhaps explains the origin of a singular instinct in
the allied group of ostriches. For several hen ostriches, at least in the
case of the American species, unite and lay first a few eggs in one nest
and then in another; and these are hatched by the males. This instinct may
probably be accounted for by the fact of the hens laying a large number of
eggs; but, as in the case of the cuckoo, at intervals of two or three days.
This instinct, however, of the American ostrich has not as yet been
perfected; for a surprising number of eggs lie strewed over the plains, so
that in one day's hunting I picked up no less than twenty lost and wasted
eggs.

Many bees are parasitic, and always lay their eggs in the nests of bees of
other kinds. This case is more remarkable than that of the cuckoo; for
these bees have not only their instincts but their structure modified in
accordance with their parasitic habits; for they do not possess the
pollen-collecting apparatus which would be necessary if they had to store
food for their own young. Some species, likewise, of Sphegidae (wasp-like
insects) are parasitic on other species; and M. Fabre has lately shown good
reason for believing that although the Tachytes nigra generally makes its
own burrow and stores it with paralysed prey for its own larvae to feed on,
yet that when this insect finds a burrow already made and stored by another
sphex, it takes advantage of the prize, and becomes for the occasion
parasitic. In this case, as with the supposed case of the cuckoo, I can
{219} see no difficulty in natural selection making an occasional habit
permanent, if of advantage to the species, and if the insect whose nest and
stored food are thus feloniously appropriated, be not thus exterminated.

_Slave-making instinct._--This remarkable instinct was first discovered in
the Formica (Polyerges) rufescens by Pierre Huber, a better observer even
than his celebrated father. This ant is absolutely dependent on its slaves;
without their aid, the species would certainly become extinct in a single
year. The males and fertile females do no work. The workers or sterile
females, though most energetic and courageous in capturing slaves, do no
other work. They are incapable of making their own nests, or of feeding
their own larvae. When the old nest is found inconvenient, and they have to
migr