22 DEVELOPMENT AND HEREDITY. 



development of the species, as the material parti- 

 cles of the brain replace each other without inter- 

 rupting the mental unity of the individual, then the 

 difficulty as to the origin of instinct would van- 

 ish, and instead of what we call the instincts of 

 the present generation we would recognise only 

 the accumulated experience of past generations of 

 ancestors, naturally adapted and made use of by 

 the intelligence which acquired it. 



We know that the material structures of the 

 brain, which are the correlates of instinct, are repro- 

 duced by the power of heredity. The question arises, 

 can we conceive of animal reproduction as reproduc- 

 ing those material changes of the brain which are 

 produced by contact with environment and which 

 are the physical presentments of what we know as 

 experience ^ The general tendency of living things 

 to reproduce themselves not only in general form, 

 but also in minute details, is too well known to need 

 a repetition of its proofs ; and since instinct and 

 intelligence are so closely interwoven in the psy- 

 chic life of animals, why should not the structural 

 changes correlated with intelligent action be repro- 

 duced by the process of inheritance as well as those 

 material conditions and changes correlated with 

 instinct .' For, as we have seen, we have reason to 

 believe that the embryonic development of instinct 



