172 INTELLIGENT BEHAVIOUR 
new departures being acquired in the course of individual life. 
This extreme case would afford an example of what we may 
term completely stereotyped behaviour. On the other hand, let 
us assume the existence of an animal with no hereditary 
definiteness of reaction, whose every act is intelligent, whose 
whole behaviour is the result of individual acquisition. This 
antithetical extreme case would afford an example of what we 
may term completely plastic behaviour. It is questionable, 
however, whether either of these extreme types occur in nature. 
What we find in our study of animal behaviour is some inter- 
mediate condition in which both factors co-operate, with a 
predominance either of stereotyped instinctive response on the 
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 unvarying constancy in the 
environment to which the stereotyped instinctive behaviour is 
adapted. Instinct without intelligence is like a barrel-organ 
constructed to play a limited number of tunes with monotonous 
precision. The music of its behaviour depends entirely on 
what the maker, heredity, has inserted in the works. But 
though a barrel-organ may suffice where a hymn-tune, a jig, a 
