It seems difficult, then, to express clearly what is the essential difference between instinctive and intelligent behaviour; and it is doubtless the case that reasoned experiments and observations are still too few to enable us to make sound deductions. But it certainly seems as if we ought to think of instinctive actions as having evolved concomitantly with the structure of the organs which effect them: they are those inheritable adaptations of behaviour which are bound up with—are indeed the same things as—inheritable adaptations of structure. In performing them the instinctively acting animal is doubtless aware of its own activity, but we must think of this awareness as being of much the same nature as our consciousness of the automatic activities of our own bodies—the rhythmic activities of the heart and respiratory organs, or the actions of our arms and legs in walking, for instance. It is knowledge of the inborn ability of the organisms to use an inborn bodily tool.

In the intelligent action we certainly see something different from this. The organ or organ-system which carries out such an action functions in a manner which is different from that for which it was evolved: the action is the conscious adaptation of the organ for some form of activity new to it, and this acquirement of activity seems to be non-inheritable—at least it is non-inheritable in the sense in which we speak of acquired characters being non-inherited. It is accompanied, while it is being acquired, by a consciousness which is deliberative, and is different from that awareness of its own activity which accompanies the acting of the instinctive animal—the knowledge that it is acting in an effective manner. It does not seem as if the animal in so acting is aware of the relation of the bodily tool to the object on which it is acting. But intelligence seems to imply more than this: it implies the knowledge of the organism that some parts of its body bear certain relations to the parts of the environment on which they are acting, and that these relations are variable ones and may be the objects of conscious choice.