EVOLUTION OF INTELLIGENCE. 113 



Having thus cleared the ground it may at once be 

 pointed out that instincts correspond to simple adapta- 

 tions fixed in definite directions ; they are formed of a 

 series of responses which must succeed each other when 

 a particular stimulus is applied and reach a predeter- 

 mined end, as surely as the piece of chocolate is yielded 

 up by the penny-in-the-slot machine when the right 

 coin is dropped in. We may pass from the simplest 

 "reflex action, such as the contraction of a muscle on 

 the application of a stimulus to the skin, to the most 

 elaborate instinct found in bees and other social insects, 

 along a series of such direct chains of reactions of gradually 

 increasing complexity. 



The scientific analysis of the behaviour of animals has 

 only begun, and as a rule we "can trace but very incom- 

 pletely the actual physico-chemical processes which 

 underlie it. But important advances have lately been 

 made in this direction, chiefly by J. Loeb, who has sug- 

 gested a physico-chemical explanation of the tropisms. 

 Tropisrn is the name given to those simple instincts, the 

 material out of which the behaviour of the lower animals 

 is to a great extent composed, which appear as direct 

 responses to external stimuli. Thus when a moth flies 

 towards a flame, or a fly to a lighted window, it is said to 

 be positively heliotropic (or phototropic) ; when a worm 

 moves away from the light it is said to be negatively 

 heliotropic. Similarly Chemotropism leads animals to 

 move either away from or towards certain substances of 

 particular chemical composition, as for instance food and 

 water ; and Geotropism leads them to move either with 

 or against the force of gravity. Stereotropism is the name 

 given to the peculiar instinct shown by certain animals 

 to place as large a part as possible of the surface of their 

 body in contact with foreign objects, and so induces them 

 to creep into narrow chinks and crannies. Now although 

 these tropisms may be built up and specialised by selec- 

 tion into elaborate instincts, adaptations for special 



(2,031) 8 



