INFLUENCE OF INTELLIGENCE ON INSTINCT 173 
hornpipe, and a funeral march exhaust all the possible require- 
ments, it is sadly lacking in musical plasticity. To obtain 
that, you must place intelligence at the keyboard that the music 
may be accommodated to a greater number of varying moods. 
It may be urged that such an illustration is, in many 
respects, obviously faulty ; and that barrel-organs do not under 
any circumstances develop into musicians. No doubt the 
illustration is faulty. But it may be questioned whether 
instinct under any circumstances develops into intelligence, any 
more than a barrel-organ into a musician. As we said at the 
beginning of this section, intelligence and instinct are in large 
degree independent though there is continual interaction 
betweén them. Completely stereotyped behaviour, in its 
theoretical perfection, is in exact adaptation to the circum- 
stances. 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 it no more contains within itself 
the potentiality of developing into plastic accommodation to 
varying circumstances, than the barrel-organ contains within 
itself the potentiality of becoming a musician. The evolution 
of intelligence is along independent lines of progress. Stereo- 
typed adaptation can never pass up into plastic accommodation. 
These two belong to independent lines of evolution ; but they 
are bound up in 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 intelligence 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 
hehaiour, it may be urged that, on theoretical :grounds, the 
claims of instinct are the stronger. No doubt the evolution 
of the two lines of development have proceeded to a large 
extent side by side ; but whereas intelligence does in a number 
