er; 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 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
migrate, it is the slaves which determine the migration, and actually
carry their masters in their jaws. So utterly helpless are the masters,
that when Huber shut up thirty of them without a slave, but with plenty
of the food which they like best, and with their larvae and pupae to
stimulate them to work, they did nothing; they could not even