The Descent of Man

It is, however, impossible to decide in many
cases whether certain social instincts have been
acquired through natural selection, or are the
indirect result of other instincts and faculties,
such as sympathy, reason, experience, and a
tendency to imitation; or again, whether they
are simply the result of long-continued habit.
So remarkable an instinct as the placing sentinels
to warn the community of danger, can
hardly have been the indirect result of any of
these faculties; it must, therefore, have been
directly acquired. On the other hand, the habit
followed by the males of some social animals of
defending the community, and of attacking
their enemies or their prey in concert, may perhaps
have originated from mutual sympathy;
but courage, and in most cases strength, must
have been previously acquired, probably
through natural selection.
Of the various instincts and habits, some are
much stronger than others; that is, some either