en stroked the plant-lice with a hair,
endeavoring thus to imitate the action of the ant's feelers, but not a
single plant-louse seemed disposed to emit the secretion. Thereafter a
single ant was admitted to their company, the insect, in Mr. Darwin's
words, appearing, "by its eager way of running about, to be well aware
what a rich flock it had discovered." The ant first stroked one aphis
and then another, each insect excreting a drop of the sweet juice "as
soon as it felt the antennae;" and "even the quite young aphides
behaved in this manner, showing that the action was instinctive, and
not the result of experience." If, as Mr. Darwin remarks, it is a
convenience for the aphides to have the sweet secretion removed, and
that "they do not excrete solely for the food of the ants," the
observation does not in any degree lessen the curious nature of the
relationship which has become established between the ants and their
neighbors, or the interesting features in ant life which have
inaugurated and perpetuated the habit.

Not less remarkable are the "slave-making" instincts of certain
species of ants. It may be safely maintained that the slave-making
habit forms a subject of more than ordinary interest not merely to
naturalists but to metaphysicians given to speculate on the origin and
acquirement of the practices of human existence. Pierre Huber, son of
the famous entomologist, was the first to describe the slave-making
instincts in a species (_Polyergus rufescens_) noted for its
predaceous instincts, and subsequent observations have shown that
other species participate in these habits. _Polyergus_ is thoroughly
dependent on slaves. Without these bonds-men it is difficult to see
how the ants could exist. Huber tells us that the workers of this
species perform no work save that of capturing slaves. Use and wont,
and the habit of depending entirely on their servitors, have produced
such changes in the structure of the ants that they are unable to help
themselves. The jaws of these ants are not adapted for work; they are
carried by their slaves from an old nest to a new one; and, more
extraordinary still, they require to be fed by their slaves, even with
plenty of food close at hand. Out of thirty of these ants placed by
Huber in a box, with some of their larvae and pupae, and a store of
honey, fifteen died in less than two days of hunger and of sheer
inability to help themselves. When, however, one of their slaves was
intro