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without exhibiting any co-ordinated behaviour in relation to a variety 
of different circumstances. Parasitism affords one of the best illustra¬ 
tions. The parasite and the host simply modify each other’s environ¬ 
ment, just as two plants growing side by side may deprive each 
other of light or moisture. There are, however, many cases in which 
such accidental or non-co-ordinated reaction may be mutually 
beneficial, as for instance in some types of symbiosis. In the relations 
between various orchids and the fungus Rhizoctonia, there has been 
found to be a continuous gradation from parasitism (harmful associ¬ 
ation) to symbiosis (beneficial, and in some cases necessary associ¬ 
ation) according to virulence or biological type of Rhizoctonia with 
which the host has been infected. This kind of symbiosis does not 
imply functional integration. The same is true of the association of 
nitrifying bacteria with the roots of leguminous plants. These phe¬ 
nomena simply mean that in the course of evolution the host has 
developed a system of physiological economy, in relation to a 
particular type of primary environment in which a certain kind of 
fungus is a necessary factor. They are examples of mutual adaptation, 
not essentially different from that between various species in an 
ecological group 1 . 
There are, however, other complex and indubitable examples of 
functional integration in which the activities of different individuals 
are involved, as for instance in colonies of social insects. In this case 
also, integration of activities is accompanied by physiological re¬ 
actions, some of which are observed and others only presumptive 
(e.g. the existence of complex stimuli and inherited reflex nervous 
mechanisms), but it is not from such reactions that the unity of the 
colony is inferred. We recognise this unity by the way in which the 
instinctive acts of the different individual insects are co-ordinated 
in the many diverse circumstances that arise in the course of the 
colony’s life. In the development of human society there is a still 
more complex example, where the activities functionally integrated 
include both instinctive and intelligent behaviour. 
We have endeavoured to establish in the preceding discussion 
that functional integration is a unique biological phenomenon. In 
the individual organism the activities are always integrated; but in 
the relations between different individuals there may be environ- 
1 In some cases however one partner may “control" the other, as in certain 
insect-bacteria associations where the female has special organs for infecting 
the ova. The fungus has in this case become incorporated in the secondary 
environment of the insect, and the reactions between them are only accidental 
so far as the fungus is concerned. 
