Variation as an Organic Function 181 
development. Finally, there are occasional cases of functional dis¬ 
sociation of the more stable activity-systems that may give rise to 
abnormal and retrogressive types. 
The process of phylogenetic evolution is not, however, analogous 
to an unravelling of the strands of a cord, but proceeds in the reverse 
direction. In the succession of generations the organism is constantly 
experiencing new reactions that are not a normal feature of somatic 
development. These may have an obvious external origin, such as 
many of the accidental and inconstant bodily movements of plants, 
or they may arise as abnormal fluctuations in internal physiological 
processes. In the higher types, fortuitous muscular movements of 
the various organs are also included. Our theory implies that these 
sporadic and ephemeral reactions become progressively co-ordinated 
into definite sequences or activities, which in turn become integrated 
with the more focal activity-systems. This co-ordination does not 
however take place fortuitously, but as a definite selective synthesis 
conditioned at each step by the physical limitations of the environ¬ 
mental factors involved 1 . 
Let us consider in illustration the evolution of a complex type of 
instinctive behaviour, such as that of various species of solitary 
wasp, which capture caterpillars, paralyse them by a sting in the 
cerebral ganglion, and then immure them in their mud cells as a 
source of food material for their own larvae, when the latter have 
hatched from the egg. We shall assume to begin with that the insect 
has already acquired a capacity for co-ordinated bodily movements 
of various kinds, and that in the course of its life such movements 
constantly occur in relation to a variety of external circumstances 2 . 
When the instinct has been perfected, a particular group of bodily 
movements has become co-ordinated in a definite sequence in relation 
to a particular set of conditions. The question we have to consider 
is how this co-ordination has been achieved. From the Darwinian 
standpoint chance movements are presumed to become co-ordinated 
in relation to chance stimuli in any direction (as a result of fortuitous 
inherited linkages in the nervous system), and the final combination 
is attained by the elimination of less efficient combinations through 
differential individual extinction in the struggle for existence. The 
1 This is of course a proximate interpretation consistent with the method 
of analysis we have adopted; it does not however preclude the possibility of 
an ultimate physiological interpretation of the same facts if they can be 
analysed completely in a different way. 
2 This implies of course that a complex type of behaviour has been already 
evolved. 
