114 THE DESCENT OF MAN. 



strong mutual affection the two former kinds, at least, 

 show on their reunion. It is curious to speculate on the 

 feelings of a dog, who will rest peacefully for hours in a 

 room with his master or any of the family, without the least 

 notice being taken of him; but if left for a short time by 

 himself, barks or howls dismally. "We will confine our at- 

 tention to the higher social animals, and pass over insects, 

 although some of these are social, and aid one another in 

 many important ways. The most common mutual service 

 in the higher animals is to warn one another of danger by 

 means of the united senses of all. Every sportsman knows, 

 as Dr. Jaeger remarks,* how difficult it is to approach 

 animals in a herd or troop. Wild horses and cattle do not, 

 I believe, make any danger-signal ; but the attitude of any 

 one of them who first discovers an enemy, warns the others. 

 Kabbits stamp loudly on the ground with their hind feet as 

 a signal ; sheep and chamois do the same with their fore 

 feet, uttering likewise a whistle. Many birds and some 

 mammals, post sentinels, which in the case of seals are saidf 

 generally to be the females. The leader of a troop of mon- 

 keys acts as the sentinel, and utters cries expressive both of 

 danger and of safety. J 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 parasites; and Brehm states that after a troop 

 of the Cercopithecus griseo-viridis has rushed through a 

 thorny brake, each monkey stretches itself on a branch, 

 and another monkey sitting by, i{ conscientiously "examines 

 its fur> and extracts every thorn or burr. 



Animals also render more important services to one 

 another; thus wolves and some other beasts of prey hunt in 

 packs, and aid one another in attacking their victims. Peli- 

 cans fish in concert. The Hamadryas baboons turn over 

 stones to find insects, etc. ; and when they come to a large 

 one, as many as can stand round, turn it over together and 



* " Die Darwin'sche Tkeorle," s. 101. 



| Mr. R. Brown in ff Proc. Zoolog. Soc.," 1868, p. 409. 



^ 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 trust- 

 worthy. For the cases of the old male baboons attacking the dogs, 

 see s. 79; and with respect to the eagles 6. 56. 



