Our knowledge of vertebrate psychology is not yet sufficient to give a history of the struggle for supremacy between instinct and intelligence, between inherited tendency and the consciousness of the individual.  But the outcome is evident; intelligence prevails, instinct wanes.  The actions of the young may be purely instinctive; it is better that they should be.  But instinct in the adult is more and more modified by intelligence gained by experience.  There is perhaps no more characteristic instinct than the habit of nest-building in birds.  And yet there are numerous instances where the structure and position of nests have been completely changed to suit new circumstances.  And the view that this habit is a pure instinct, unmodified by intelligence, has been disproved by Mr. Wallace.  But while size of brain, keenness of sense-organs, and length of life may be rightly emphasized as the most important elements in the development of vertebrate intelligence, the importance of the appendages should never be forgotten.  Cats seem to have acquired certain accomplishments—­opening doors, ringing door-bells, etc.—­never attained by the more intelligent dog, mainly because of the greater mobility and better powers of grasping of the forepaws.  The elephant has its trunk and the ape its hand.  The power of handling and the increased size of the brain aided each other in a common advance.

The teachableness of mammals is also a sign of high intelligence.  The young are often taught by the parent, a dim foreshadowing of the human family relation.  And we notice this capacity in domestic animals because of its practical value to man.  And here, too, we notice the difference between individuals, which fails in instinct.  All spiders of the same species build and hunt alike, although differences caused by the moulding influence of intelligence will probably be here discovered.  But among individual dogs and horses we find all degrees of intelligence from absolute stupidity to high intelligence.  And many mammals are slandered grievously by man.  The pig is not stupid, far from it.

Still only in man does intelligence reign supreme and clearly show its innate powers.  But even in man certain realms, like those of the internal organs, are rarely invaded by consciousness, but are normally left to the control of reflex action.  These actions go on better without the interference of consciousness.

But other lines of action are relegated as rapidly as possible to the same control.  We learn to walk by a conscious effort to take each step; afterward we take each step automatically, and think only whither we wish to go.  We learn by conscious effort to talk and write, to sing, or play the piano.  Afterward we frame each letter or note automatically, and think only of the idea and its expression.

So also in our moral and spiritual nature.[A]