INFLUENCE OF INTELLIGENCE ON INSTINCT 173 
one hand, or of plastic intelligent acquisition on the other 
hand. And in the latter case, as such behaviour approaches its 
ideal limits, we have modifiability under the circumstances of 
individual life at its maximum. 
The evolution of intelligence as such runs parallel with the 
evolution of plastic behaviour ; and this plasticity is necessi- 
tated by the variety and the complexity of the conditions of 
life—a variety and a complexity requiring many subtle modifi- 
cations of response to enable the behaviour to reach accommoda- 
tion to the changeful exigencies of diverse circumstances. To 
meet constant and relatively fixed conditions stereotyped 
instinctive responses suffice ; and the elimination under natural 
selection of those individuals which fail to respond in fixed 
ways by specially adaptive behaviour tends to render definite 
the hereditary channels of nervous intercommunication. An 
inherited system of no little complexity may thus be evolved ; 
of which we have seen examples in our study of instinctive 
behaviour. But the essential condition of the successful 
working of such a system is constancy in the environment 
to which the instinctive behaviour is adapted. Completely 
stereotyped behaviour, in its theoretical perfection, is in 
exact adaptation to the circumstances. Where instincts are 
only relatively perfect, further adaptation is secured through 
congenital variation and the survival of the individuals in 
which the behaviour is better adapted to the comparatively 
invariable circumstances. ‘This is one line of evolution. But 
the evolution of intelligence is along independent lines of pro- 
gress. Both, however, result from the functional activity of 
the same nervous system, they jointly determine the behaviour, 
they interact not only in the course of individual life but in the 
process of evolution, and they are both subject to the incidence 
of natural selection, which can determine whether the one line 
or the other shall preponderate—whether instinct or intelli- 
gence shall dominate behaviour. 
If an answer must be given to the question whether instinct 
or intelligence has priority in the course of the evolution of 
behaviour, it may be urged that, on theoretical grounds, the 
