either of the same or of a distinct species, is not very uncommon with
the Gallinace ; 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
Sphegid (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 larv 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
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 larv . When the old nest is found
inconvenient, and they have to migrate, it is the slaves which
determine the migration, and actually carry their masters in their
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