18 DEVELOPMENT AND HEREDITY. 



there is a period in the growth of the bird's brain 

 when there are nervous fibres in the base of the 

 brain while it is roofed over merely by a cellular 

 membrane, — thus reproducing a condition of the 

 brain which is characteristic of the majority of 

 fishes throughout their lives. The embryonic devel- 

 opment of the vertebrate brain corresponds closely 

 to what the study of morphology has shown to be 

 its probable phylogenetic development. 



But whether we can or cannot infer the psychical 

 development of the embryo from its physical develop- 

 ment, we can at least compare the psychical result 

 of the embryonic development with the psychical 

 result of the later development. And from the 

 similarity of the two results we can infer a similarity 

 in the processes which produced them. As the 

 result of the embryonic development we have in- 

 stinct ; and as the result of the development which 

 takes place in later life we have that intelligence 

 which is derived from perception and experience. 

 There is such a close and constant interaction be- 

 tween instinct and intelligence, that the material 

 changes accompanying the two phases of psychic 

 activity must often form parts of an uninterrupted 

 series, as closely united as are in us the brain- 

 changes accompanying an unbroken train of thought. 

 An action may have an instinct for its main cause. 



