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 subsequent 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 circumstances. M. Fabre describes how a _Sphex_, one of the solitary wasps, instinctively draws its prey, a grasshopper, into the burrow by its antennæ. 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 accommodation 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 grasshopper was then left. Intelligence did not seem equal to meeting the altered conditions presented by the maimed grasshopper. 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 existence of an animal whose every act is instinctive, whose whole behaviour is marked out in strictly hereditary lines, no 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 intermediate 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.