few pupae of
the yellow ant (_F. flava_), a courageous and pugnacious little
species, were placed within the reach of the slave-making
_Sanguineas_. A like chance presented with the pupae of their slave
race was eagerly seized, and the chrysalides carried off. The pupae of
the yellow ants, however, were not merely left untouched, but the
slave-makers exhibited every system of terror and alarm at the sight
of the chrysalides of their yellow neighbors. Such an instance
demonstrates the existence not merely of perception but also of the
memory of past experience, probably of not over agreeable kind, of
encounters with the yellow ants. When, on the contrary, a nest of the
slaves is attacked, the _Sanguineas_ are both bold and wary. Mr.
Darwin traced a long file of _Sanguineas_ for forty yards backward to
a clump of heath, whence he perceived the last of the invaders
marching homeward with a slave pupa in its mouth. Two or three
individuals of the attacked and desolate nest were rushing about in
wild despair, and "one," adds Mr. Darwin, "was perched motionless,
with its own pupa in its mouth, on the top of a spray of heath, an
image of despair over its ravaged home." The picture thus drawn is not
the less eloquent because its subject is drawn from lower existence;
although the pains and sorrows of ant life may not legitimately be
judged by the standard of human woe.

The explanation of the slave-making instinct in ants begins with the
recognition of the fact that many ants, not slave-makers, store up
pupae of other species for food. If we suppose that some of the pupae,
originally acquired through a cannibal-like instinct, came to maturity
within the nest of their captors, and in virtue of their own inherited
instincts engaged in the work of the hive, we may conceive of a
rational beginning of the slave-making instinct. If, further, the
captors learned to appreciate the labors of their captives, as
lightening their own work, the habit of collecting pupae as slaves
might succeed and supersede that of collecting them for food. In any
case, we should require to postulate on the part of the slave-makers a
degree of instinct altogether unusual in insects, or, indeed, in
higher animals; but that such instinct is developed in ants other than
slave-makers admits of no dispute. The strengthening, through
repetition, of a habit useful to the species may thus be credited with
the beginning of the practice of slavery amongst ants; whil