The Descent of Man

first discovers an enemy, warns the others.
Rabbits stamp loudly on the ground with their
hind-feet as a signal: sheep and chamois do the
same with their forefeet, uttering likewise a
whistle. Many birds, and some mammals, post
sentinels, which in the case of seals are said (8.
Mr. R. Brown in 'Proc. Zoolog. Soc.' 1868, p.
409.) generally to be the females. The leader of
a troop of monkeys acts as the sentinel, and
utters cries expressive both of danger and of
safety. (9. Brehm, 'Thierleben,' B. i. 1864, s. 52,
79. For the case of the monkeys extracting
thorns from each other, see s. 54. With respect
to the Hamadryas turning over stones, the fact
is given (s. 76), on the evidence of Alvarez,
whose observations Brehm thinks quite trustworthy.
For the cases of the old male baboons
attacking the dogs, see s. 79; and with respect to
the eagle, s. 56.) Social animals perform many
little services for each other: horses nibble, and
cows lick each other, on any spot which itches:
monkeys search each other for external parasi-