INFLUENCE OF INTELLIGENCE ON INSTINCT 171 
conception of the matter in the clear light of a working 
hypothesis. 
The amount of what we may term “ modifiability ” by use 
differs a good deal in the several organs and tissues. The 
teeth of carnivora and the antlers of deer may be cited as 
structures in which the conditioning effects of use form a 
relatively unimportant factor. On the other hand, the nervous 
system, with which we are here primarily concerned, is of all 
animal structures that in which what is acquired may attain 
the greatest importance in the successful conduct of life; the 
nature and the range of behaviour affording an index of the 
amount of modifiability in this respect. 
We have already seen that instinctive behaviour is primarily 
a matter of the first occasion on which any given action is 
performed, and that many instinctive acts are subject to sub- 
sequent modification in the light of the experience gained 
during the early performances. The range of such modification 
varies both in different animals and also with respect to 
different modes of behaviour in the same animal. The more 
fixed and deeply rooted an instinct the less readily does 
intelligence obtain a hold on it, so as to direct the behaviour 
into new channels of better accommodation to the circum- 
stances. M. Fabre describes how a Sphex, one of the solitary 
wasps, instinctively draws its prey, a grasshopper, into the 
burrow by its antennee. When these were cut off the wasp 
pulled the grasshopper in by the jaw appendages ; but when 
these were removed she seemed incapable of further accom- 
modation to the unusual circumstances. It would seem an 
easy and obvious application of intelligence to seize the prey by 
one of the forelegs. But this was not done; and the grass- 
hopper was then left. Intelligence did not seem equal to 
meeting the altered conditions presented by the maimed grass- 
hopper. Still, there was some modification of the normal 
instinctive behaviour ; and, as Dr. Peckham has shown, there 
may be more than Fabre noted. Let us assume the exist- 
ence of an animal whose every act is instinctive, whose 
whole behaviour is marked out in strictly hereditary lines, no 
