1898.] on Instinct and Intelligence in Animals. 577 



behaviour can only give us information that new directing threads 

 have been introduced. The psychology of association can only 

 indicate which pins have been connected by linking threads. Even 

 such researches as those of Flechsig can at present do no more than 

 supplement the psychological conclusion by general anatomical 

 evidence. Of the details of brain modification by the formation of 

 association fibres we are still profoundly ignorant. 



Nor when we turn from the narrower to the wider point of view are 

 we in better case. We are forced to content ourselves with those 

 generalities which are the makeshift of imperfect knowledge. Still 

 even such generalities are of use in showing the direction in which 

 more exact information is to be sought. And we can, perhaps, best 

 express the net result of acquired modification of brain-structure by 

 saying that every item of experience makes the animal a new being, 

 with new reactive tendencies. The sparrows, which yesterday were 

 unaffected by the stealthy approach of the cat, garrulously scatter to- 

 day, because they are not the same simple-minded sparrows that they 

 were. The chick comes into the world possessed of certain instinctive 

 tendencies, with certain hereditary directing threads. But at the touch 

 of experience its needs are modified or further defined. New con- 

 necting threads are woven in the brain. On the congenital basis has 

 been built an acquired disposition. The chick is other than it was, 

 and reacts to old stimuli with new modes of behaviour. 



In its early days, the developing animal is reading the paragraph 

 of life. Every sentence mastered is built into the tissue of experience, 

 and leaves its impress on the plastic yet retentive brain. By dint of 

 repetition the results of acquisition become more and more firmly in- 

 grained. Habits are generated, and habit becomes second nature. 

 The organism which, to begin with, was a creature of congenital im- 

 pulse and reaction, becomes more and more a creature of acquired 

 habits. It is a new being, but one with needs not less imperious than 

 those with which it was congenitally endowed. 



All of this is trite and familiar enough. But it will serve its 

 purpose if it help us to realise how large a share acquired characters 

 take in the development of behaviour in the higher animals, and how 

 fundamentally important is the plasticity of brain-tissue, and its re- 

 tentiveness of the modifications which are impressed on its yielding 

 substance. 



Such being the relations of intelligence to instinct in the indi- 

 vidual, what are their relations in the evolution of the race? Granting 

 that instinctive responses are definite through heredity, how has this 

 definiteness been brought about ? Has it been through natural 

 selection ? Or are the acquired modifications of one generation trans- 

 mitted through heredity to the next ? Is instinct inherited habit ? 

 Mr. Herbert Spencer has long advocated and still advocates the 

 latter view ; while Mr. A. E. Wallace attributes instinct entirely 

 to natural selection. Darwin, who wrote before the transmission of 

 acquired characters was seriously questioned, admitted both factors. 



