The contrary is the case with the carnivora; they are well armed, and need ample space in which to hunt down their prey, so that it is to their advantage to live in isolation, except in those cases already mentioned, where they associate together for a difficult chase, or for defence against a dangerous enemy.[174] ----- Footnote 174: Herbert Spencer, _Psychology_, ii. § 503 _et seq._ ----- We may add that there are animals which, as they find it to their advantage or otherwise, live alternately in communities or isolated. “Certain sociable birds in Australia build bowers of branches, where they assemble in great numbers during the day. In pairing-time the society is broken up, and each couple retires by itself to construct its own separate nest. While the temporary families last there is no longer any assemblage, nor any life in common; it only begins again when the young are able to try their wings. This is only one of numberless examples which might be mentioned.”[175] ----- Footnote 175: Houssay, _Revue philosophique_, May 1893, p. 487. ----- In short, gregarious life depends on stature, strength, means of defence, kind and distribution of food, and mode of propagation. Derived from necessity, this habit of life in common creates a solidarity which is not mechanical and external, but psychological: the sight, the touch, the smell of his companions constitute in each individual a part of his own consciousness, of which he feels the want in its absence; the distressed state and the lamentations of an animal separated by chance from the herd are well known. Here a disputed question suggests itself. It has been already settled by implication in the course of the statements already made, but it cannot be thus treated retrospectively and merely in passing. For the moment I may confine myself to indicating it. If we compare family societies and gregarious societies, what relation is there between them? We find ourselves in presence of two opinions or theories—one in favour of unity, the other of duality. The first, the most ancient and widespread, derives social life from domestic life. The family is the social molecule: by its increase are formed aggregates of a more or less complex character, whose life in common creates a solidarity and an exchange of services—_i.e._, the conditions of a community. The second admits two groups of irreducible feelings and tendencies, mutually independent, though there are points of contact. The social instinct is not derived from the domestic feelings, while the latter are not derived from the social feelings. They are distinct by nature, having their respective sources in the attraction of like for like, irrespective of sex, and in the sexual appetite and the development of the tender emotions.