Variation as an Organic Function 169 
contrivances that insects construct for capturing and preserving 
food. The most complex example of all—human society—is charac¬ 
terised by the fact that this external economic machinery is of 
paramount importance, and upon its stability and efficiency the life 
of the majority of the members of a highly developed community 
is ultimately dependent. In all these cases, the persistence of the 
important structures and functions is due mainly to a particular 
series of instinctive and intelligent acts having followed each other 
in a definite order. There may indeed be a very large number of 
different types of physiological economy corresponding to different 
types of activity-systems, but in each case it is generally possible 
to discover certain stable reactions that are limiting factors in 
somatic equilibrium, and that can only vary within certain restricted 
limits without throwing the whole machinery of somatic life com¬ 
pletely out of gear, unless there are compensative readjustments in 
other parts of the organism. 
The character of the environmental reaction-complex is, however, 
also partly determined by the uncontrolled primary environment. 
And as this is a variable factor the maintenance of a stable and 
efficient organism necessitates a certain elasticity in the inherited 
sequence of events. This capacity for somatic readjustment with 
respect to variations in the primary environment is generally referred 
to as “functional adaptation,” although the term “somatic equi¬ 
libration ” is perhaps a less contentious one. In the simplest cases, 
such as the compensative readjustments between root-absorption 
and transpiration in plants, or between the amount of assimilatory 
tissue in the leaves and the intensity of sunlight, the reactions 
involved are internal and physiological. The same is true of the 
slightly more complex example of the compensative hypertrophy 
of various glands in the bodies of animals when others known or 
presumed to have a similar function have undergone excision or 
atrophy. There are, however, in the higher types examples of onto¬ 
genetic readjustments, the mechanism of which is not so obviously 
physiological, as in the slight variations in the instinctive behaviour 
of insects conditioned by inconstant external factors, or in the most 
complex example of all, the intelligent acts of the higher animals 
and man. Another very specialised type of functional adaptation 
that cannot be discussed here is the non-rational changes in social 
conduct that are characteristic of the progressive development of 
human society, and which affect so profoundly the economic structure 
of the group. 
