226 SOCIAL BEHAVIOUR 
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 drives 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. 
“Tt 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 pro- 
bable 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 
