THE ANALYSIS OF BEHAVIOUR 



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latter contracts, lifting the weight against the resistance of 

 gravity. Do this again and again, and the same effect follows. 

 By-and-by the muscle, or the nerve terminations in it, will 

 become fatigued, but until this happens an electric stimulus will 

 always elicit the same muscular 

 contraction or response. Here, 

 also, there is, perhaps, strict 

 determinism. 



Tropisms in Green Plants. 



When a seed germinates, two 

 structures grow out from it. One 

 of these is the original root and 

 the other is the original shoot. 

 The former always grows down 

 into the soil, and the latter grows 

 upwards into the air. When a 

 green plant is placed in front of 

 a window, the leaves tend to 



turn towards the source of the light. The leaves of a tree 

 tend always to place themselves so that their flat surfaces 

 are perpendicular to the principal direction from which the 

 light comes. All these effects are called " tropisms," or directed 

 growth movements of fixed organisms. The tissues of the 

 plant have the general power of growth, but the stimulus of 

 light falling on them is a directive agency, which causes growth 

 to take place in one direction rather than any others. Such 

 directed growth movements in response to the stimulus of light 

 are called heliotropisms. They are invariable and subject to 

 determinism, just as are the deviation of the compass needle 

 or the contraction of the isolated frog's muscle when the nerve 

 supplying it is stimulated. 



Taxis in the Lower Animals. Chip off some barnacles 

 (Balanus) from the stones on the beach between tide- marks 

 during the last week of March or the first ones of April, and place 

 them in a soup-plate containing clean sea-water. In a short time 

 the embryos contained in the reproductive organs will hatch out 

 and swim about in the water. Examine the latter in a feeble, 

 diffused light, and it will be seen that the larvae swim about at 

 random and in no particular direction; but place a lamp close 

 to one side of the vessel, and it will be seen that they swim 



