mbiosis, to distinguish it from the simple
symbiosis that obtains between individual organisms of different species
and the intermediate form of symbiosis exhibited by individual organisms
that live in ant or termite colonies.

The researches of the past forty years have brought to light a
remarkable array of instances of social symbiosis, varying so much in
intimacy and complexity that it is possible to construct a series
ranging from mere simultaneous occupancy of a very narrow ethological
station, or mere contiguity of domicile, to an actual fusion, involving
the vital dependence or parasitism of a colony of one species on that of
another. Such a series is, of course, purely conceptual and does not
represent the actual course of development in nature, where, as in the
animal and vegetable kingdoms in general, development has not followed a
simple linear course, but has branched out repeatedly and terminated in
the varied types at the present time.

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 greate