Even if we admit the transformist hypothesis, zoological evolution has not proceeded in a straight line. This point, it is true, is contested, but it is all the more important to remember, because the development of the organisation and that of the social instincts do not always go on _pari passu_. Thus the social aptitudes of ants and bees are far superior to those of certain mammals considered of a far higher type of organisation. Without troubling ourselves, therefore, about the frequent disagreements between zoological taxinomy and sociological psychology, we shall follow the ascending march of the social instinct, no matter in what order or class or at what point of the genealogical tree it shows itself. We thus find four principal forms of animal societies: at the lowest stage, those founded on nutrition; further on, those based on reproduction; then, unstable, gregarious societies; and finally, societies with a stable and complete organisation.[168] Some special question will be put with regard to each of them, so as to show us the social question under some one particular aspect. ----- Footnote 168: For the general study of this question see Espinas, _Les Sociétés Animales_, 2nd ed. (1878), and Ed. Perrier, _Les Colonies Animales_. ----- “The idea of a society,” says Espinas, “is that of permanent co-operation between separate living beings, engaged in the same action” (_op. cit._, p. 157). The character of permanence even is not necessary for the inferior forms; there are temporary societies differing _in toto_ from those heterogeneous, fortuitous, momentary aggregates which we call crowds. Reciprocity and solidarity are the two fundamental conditions, a fact which excludes from human and animal societies two forms somewhat approximating to them: parasites, in whose case there is no reciprocity, and who show a modified form of the struggle for life; and messmates, where community of life, though it involves no injurious action, likewise implies no helpful one. I. In animal societies founded on nutrition, it is this function which constitutes the social tie; the individuals composing it are attached to one another in a permanent manner, from their birth onwards, and the nutritive liquid circulates from one to another, thus establishing a material community. It is found in the hydroid polypes, the Bryozoa and the Tunicata. As examples of the superior forms, we may quote the hydractinia, composed of individuals each of which has its own special and exclusive function: some, that of feeding; others, that of feeling and exploring; others, of defending the colony; others, again, of reproducing it—the last-named being divided into males and females. The siphonophora present an analogous division of labour, and the community, over a metre in length, suspended to a floating bladder, executes well co-ordinated collective movements.