Baboons (_Cynocephali_) who usually live in the midst of rocks protect their retreat by rolling very heavy blocks on to their aggressors, or by forcibly throwing stones about the size of the fist. As these bands may contain from a hundred to one hundred and fifty individuals, it is a veritable hail of stones of all sizes which they roll down from the heights of the mountains where they find shelter. _Sentinels._--Not only do Apes know how to face danger or to avoid it by a prudent flight, but they also seek to foresee it, and to avoid exposing themselves to it. A troop of Apes, according to Brehm, generally places the leadership in the hands of a robust and experienced male. This primitive royalty is founded partly on the confidence inspired by an old chief, and partly by the fear inspired by his muscular arms and ferocious canine teeth. (Fig. 9.) He gives himself a great deal of trouble for the security of his subjects, and does not abuse the authority which he possesses. Always at the head, he leaps from branch to branch, and the band follows him. From time to time he scales a tall tree, and from its heights scrutinises the neighbourhood. If he discovers nothing suspicious a particular guttural grunt gives information to his companions. If, on the contrary, he perceives some danger he warns them by another cry, and all draw in ready to follow him in his retreat, which he directs in the same way as he guided the forward march. Apes are not alone in relying on the experience of one of their members. Many other animals act in the same way: antelopes, gazelles, elephants, who advance in troops always conducted by an old male or female who knows all the forest paths, all the places favourable to pasture, and all the regions which must be avoided. [Illustration: FIG. 9.] Others, more democratic, instead of giving up the care of their safety to one individual, which cannot be done without abdicating some degree of individual independence, dispose around the place which they occupy a certain number of sentinels charged to watch over the common safety. This custom exists among prairie dogs, moufflons, crows, paroquets, and a great many other animals. The sentinels of the crows are not only always on the watch, but they are extremely discriminating; they do not give a warning at the wrong time. It is certain that these birds can distinguish a man armed with a gun from another who merely carries a stick, and they allow the second to approach much nearer than the first before giving the alarm. Paroquets of all species live in joyous and noisy bands. After having passed the night on the same tree they disperse in the neighbourhood, not without having first posted watchers here and there, and they are very attentive to their cries and indications.