In its lowest degree it consists of accidental and unstable assemblages which are, as it were, an attempt at life in common. Every one knows that certain pelagic animals travel in vast numbers, their course being determined by the temperature of the water or the direction of the currents. We also know what happens in the migrations of processionary caterpillars, of crickets, and more especially of birds. Numerous species of animals assemble together in the morning and evening to sing, utter their various cries, pursue each other and gambol about, living dispersed at other times. This shows, says Espinas, “a latent social tendency, always ready to show itself when not combated by any other tendency.” Higher still, we find assemblages of variable duration, but voluntarily formed and maintained, in view of a common aim. They have all the characteristics of a society—community of effort, synergy, reciprocity of services. Darwin[173] has given many examples of this: Pelicans fish in concert, and close in round their prey like a living net; wolves and wild dogs hunt in packs, and help each other to attack their victims. These communities are to some extent accidental and unstable, and may come to an end in a final competition for the sharing of the spoil. Much more stable are those which have the common defence for their aim: rabbits warn one another of danger; many mammals and birds place sentinels (it is well known how difficult it is to approach a herd or drove of animals); monkeys remove vermin or take out thorns from one another’s skins, form a chain to cross the gap between two trees, unite their forces to raise a heavy stone, and finally, gathered into bands under the direction of a leader, they defend themselves energetically and risk their lives to save their companions. We might enumerate endless facts of this kind. No doubt, we have not yet found the permanent organisation, the fixed division of labour, the continuity, which are peculiar to the higher animal societies; but the instability and intermittence of these social forms help us to understand why they exist and whence they originate. ----- Footnote 173: _Descent of Man_, chap. iii. See also Espinas, _op. cit._, sec. iv. ----- Social tendencies are derived from sympathy; they arise in determinate conditions. The facts already given supply the answer to the questions: how do they arise? what is their source? They arise from the nature of things, from the conditions of the animal’s existence; they are not based on pleasure, but on the unconscious affirmation of the will to live; they are auxiliary to the instinct of conservation. Society, as Spencer justly remarks, is founded on its own desire—_i.e._, on an instinct. The gregarious life, as this writer has shown in detail, predominates among the herbivora and graminivora, who, as a rule, being ill armed for strife and finding food in abundance, find it to their advantage to live in herds.