It is convenient to follow the European writers, von Hagens, Forel, Wasmann, and others, in grouping all the cases of social symbiosis under two heads, the compound nests and the mixed colonies. Different species of ants or of ants and termites are said to form compound nests when their galleries are merely contiguous or actually interpenetrate and open into one another, although the colonies which inhabit them bring up their respective offspring in different apartments. In mixed colonies, on the other hand, which, in a state of nature, can be formed only by species of ants of close taxonomic affinities, the insects live together in a single nest and bring up their young in common. Although each of these categories comprises a number of dissimilar types of social symbiosis, and although it is possible, under certain circumstances, as will be shown in the sequel, to convert a compound nest into a mixed colony, the distinction is nevertheless fundamental. It must be admitted, however, that both types depend in last analysis on the dependent, adoption-seeking instincts of the queen ant and on the remarkable plasticity which enables allied species and genera to live in very close proximity to one another. By a strange paradox these peculiarities have been produced in the struggle for existence, although this struggle is severer among different species of ants than between ants and other organisms. As Forel says: "The greatest enemies of ants are other ants, just as the greatest enemies of men are other men." 3. The Taming and the Domestication of Animals[83]