What is the origin of these strange co-partnerships—for there are others—which we find existing between ants of two different species living in the same or in one double nest? As we know, the different species of ants are commonly very hostile to one another; and for any to enter the nest of some other one is to court destruction, if they be not the stronger party. Nay, they dare not even enter a strange nest of their own species. It seems probable, therefore—this, at least, is my own view of it—that such friendly cohabitation has come about through the channel, not of peace, but of war—through successful encroachments which, from being unavoidable, have come gradually to be less and less resented, till the two parties, mingling freely, have learnt to live on other terms. Now there are ants which live, like ogres, in the nests of other species, preying upon their eggs and young. Such a one is _Solenopsis fugax_—to whom we will come presently—but it is perhaps even more interesting to find in species between whom relations of a similar nature to those which we have been considering exist, occasional slight traces of a mutual hostility. As between the _Leptos_ and _Myrmicas_ indeed this has only as yet been noticed, very faintly, on the part of the former; but in regard to another pair who live together—our great wood-ant, namely, _Formica rufa_ and tiny little _Formicoxenus nitidulus_—Professor Wheeler remarks: “On one occasion in one of my artificial nests, in which the ants had previously lived on good terms with one another, I saw a _Formica_ touching a _Formicoxenus_ with her antennæ and menacing her with her mandibles, but she departed without even attempting to seize her. In the same nest I found a _Formicoxenus_ which had seized the leg of a _Formica_ in its mandibles and had died in this position.”[57] Other observers, too, have from time to time—but only very occasionally—noticed facts of the same sort. Through such exceptional slight indications we may perhaps see, “as in a glass darkly,” what things were at the beginning.