INFLUENCE OF INTELLIGENCE ON INSTINCT 175 
But though under constant and uniform changes in the 
environment the net result would be onty a guided variation 
of the original instinct, under more variable and indefinitely 
changing circumstances the result would be different. The 
higher animals exhibit an intelligent plasticity which enables 
them to meet the requirements of the more complex environ- 
ment into which their wider life has risen ; for evolution lifts 
the animal from narrower into progressively wider spheres of 
activity and behaviour, so that its environment becomes 
relatively more complex. Here stereotyped behaviour would 
be rather a hindrance than an advantage. The winning 
animal in life’s struggle would be the one in which behaviour 
was most rapidly and most surely modified to meet particular 
needs—the one in which the teachings of experience were most 
promptly utilized in effective action. ‘The inevitable tendency 
of the evolution of intelligence must be disintegration of the 
stereotyped modes of behaviour and the dissolution of instinct. 
Natural selection, which under a uniform and constant 
environment leads to the survival of relatively fixed and 
definite modes of response, under an environment presenting 
a wide range of possibilities leads to the survival of plastic 
accominodation through intelligence. It is not that intelligence 
has any direct influence tending to undermine the hereditary 
foundations of instinct, for acquired plasticity is not inherited 
as such; ib is rather that when the stereotyped and the 
plastic are pitted against each other in the struggle for 
existence in the wider, freer, and more varied life of the 
higher animals the plastic survives and the stereotyped 
succumbs. 
Imperfect as is our present knowledge of the manner 
in which the nervous connections implied in psychological 
associations are established, there can be no question that 
they are acquired in the course of individual life; they are 
modifications of nervous structure due to a special mode of 
use under the conditions of experience. Here, then, in the 
case of the nervous system, as in that of the bodily organs 
before mentioned, two co-operating factors determine the limits 
