But the nature of the functions and the form of the organs are inseparable. If two beings are endowed with functions that necessarily combine, they are also endowed with organs, if not similar, at least corresponding. And these beings with like or corresponding organs are either of the same species or of very nearly the same species.
However, circumstances may be met where two beings with quite different organs and belonging even to widely remote species may be accidentally and at a single point useful to each other. A habitual relation may be established between their activities, but only on this one point, and in the time limits in which the usefulness exists. Such a case gives the occasion, if not for a society, at least for an association; that is to say, a union less necessary, less strict, less durable, may find its origin in such a meeting. In other words, beside the normal societies formed of elements specifically alike, which cannot exist without each other, there will be room for more accidental groupings, formed of elements more or less specifically unlike, which convenience unites and not necessity. We will commence with a study of the latter.
To society the most alien relations of two living beings which can be produced are those of the predator and his prey. In general, the predator is bulkier than his prey, since he overcomes him and devours him. Yet smaller ones sometimes attack larger creatures, consuming them, however, by instalments, and letting them live that they themselves may live on them as long as possible. In such a case they are forced to remain for a longer or a shorter time attached to the body of their victim, carried about by it wherever the vicissitudes of its life lead them. Such animals have received the name of parasites. Parasitism forms the line inside of which our subject begins; for if one can imagine that the parasite, instead of feeding on the animal from whom he draws his subsistence, is content to live on the remains of the other's meals, one will find himself in the presence, not yet of an actual society, but of half the conditions of a society; that is to say, a relation between two beings such that, all antagonism ceasing, one of the two is useful to the other. Such is commensalism. However, this association does not yet offer the essential element of all society, co-operation. There is co-operation when the commensal is not less useful to his host than the latter is to the commensal himself, when the two are concerned in living in a reciprocal relation and in developing their double activity in corresponding ways toward a single and an identical goal. One has given to this mode of activity the name of mutualism. Domestication is only one form of it. Parasitism, commensalism, mutualism, exist with animals among the different species.