SENSES, INSTINCTS, AND INTELLIGENCE 335 



flame. When carrier-pigeons make from a distance for 

 their home, they may be utihsing some tropism as well as 

 the results of geographical education. When a migrant 

 bird flies in a constant direction across the pathless sea it 

 may be automatically obeying an engrained tropism. 



Fig. 49. — Diagram illustrating the inclined plane of animal behaviour. 

 I., ver/ simple actions; i, reactions; li., simple tentatives ; 2, fixed 

 reactions ; 111., trial and error experiments ; 3, simple reflexes ; iv. v. VI., 

 experiments, experiential learning and associative learning ; 4, 5, 6, more 

 complex reflex actions, tropisms, and enregistered rhythms ; vii., intelli- 

 gent behaviour ; 7, 8, simple instincts and chain instincts ; vni., rational 

 conduct (confined to man) ; 9, habituated intelligent behaviour ; 10, un- 

 conscious cerebration. Each line is drawn double, the convex side 

 indicating biosis or physiological processes, the concave side indicating 

 psychosis or mental processes. As we ascend the series the psychical 

 aspect becomes more and more obvious. 



The well-known Planarian worm called Convoluta 

 ascends to the surface of the sand whenever the tide goes 

 out, and disappears below the surface whenever the tide 

 comes in. This is more than reaction to stimuli, for the 

 worms will manifest the same periodicity in a tideless 

 aquarium, and will continue doing so for some time. There 

 are many of these enregistered rhythms in the internal 

 economy of animals. It is probable that they account in 

 part for the restlessness of migrant birds when the time 

 comes for their journey. 



