Variation as an Organic Function 167 
mental reactions with or without functional integration. It is 
possible of course to assume an ultimate physiological interpretation 
of even the most complex examples (such as insect colonies and 
human society). But such an assumption does not help us to under¬ 
stand the proximate significance of these various phenomena in 
development and evolution. On the other hand, to describe the 
latter in psychological terms fails to correlate them with the inferior 
activities of the organism with which they are intimately associated. 
From our standpoint, all cases of functional integration are qualita¬ 
tively similar and must be regarded as biological phenomena of one 
type, which may be defined in terms of the co-ordination of activity- 
systems here adopted. 
In the relations between different organisms there may, however, 
be a continuous gradation from the extreme cases of physiological 
reaction with no integration (parasitism or mutual adaptation), to 
the most highly integrated social groups. The various examples of 
colonial algae, polyps, etc., probably occupy an intermediate position. 
A simile will perhaps serve to make this important distinction 
clearer. The life of an organism, or of several associated organisms, 
may be compared with a skein of variously-coloured threads. Each 
thread represents an activity ( i.e . some particular sequence of bio¬ 
logical events), while the cross-section of the whole skein represents 
the environmental reaction-complex at any moment. The degree of 
functional integration then becomes represented by the constancy 
in the relative position of the different threads towards each other 
at different positions in the skein. If it is found to consist of a single 
twisted bundle of threads, it represents a biological individual; if a 
number of separate interlacing bundles which nowhere run parallel 
for any appreciable distance, it represents different reacting indi¬ 
viduals the activities of which are not functionally integrated. 
There may of course be an indefinite number of intermediate cases. 
Conversely, there are markedly abnormal types of behaviour on 
the part of biological individuals that must be ascribed to the 
functional dissociation of stable activity-systems which have been 
built up in the course of evolution. As examples may be mentioned: 
cytological aberrations in particular organic tissues; foetal mon¬ 
strosities ; and the more complex cases of functional mental diseases 
in the higher animals and man. In some instances it may be possible 
to point out particular environmental factors as the proximate 
cause of the abnormality, but not in others. The interpretation in 
terms of functional dissociation is, however, applicable to all cases. 
