“Animals of many kinds,” said Darwin,[108] “are social; every one must have noticed how miserable horses, dogs, sheep, etc., are when separated from their companions. The most common mutual service in the higher animals is to warn one another of danger. Every sportsman knows 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. Rabbits 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. The leader of a troop of monkeys acts as such, and utters cries expressive both of danger and of safety. Social animals perform many little services for each other: horses nibble, and cows lick each other; monkeys search each other for external parasites, and are said to remove thorns and burrs. Social animals mutually defend each other. Bull bisons in North America, when there is danger drive the cows and calves into the middle of the herd, whilst they defend the outside. Among baboons the old males come forward to the attack. Wolves hunt in packs; and pelicans fish in concert. “It has often been assumed,” continues Darwin, “that animals were in the first place rendered social, and that they feel as a consequence uncomfortable when separated from each other, and comfortable whilst together; but it is a more probable view that these sensations were first developed, in order that those animals which would profit by living in society should be induced to live together, in the same manner as the sense of hunger and the pleasure of eating were, no doubt, first acquired in order to induce animals to eat. The feeling of pleasure from society is probably an extension of the parental and filial affections, since the social instinct seems to be developed by the young remaining long with their parents; and this extension may be attributed in part to habit, but chiefly to natural selection. With those animals which were benefited by living in close association, the individuals which took the greatest pleasure in society would best escape various dangers; while those which cared least for their comrades, and lived solitary, would perish in greater numbers. In however complex a manner the feeling of sympathy may have originated, as it is one of high importance to all those animals which aid and defend one another, it will have been increased through natural selection; for those communities which included the greatest number of the most sympathetic members would flourish best, and rear the greatest number of offspring.”