44 PLANT-ANIMALS [CH. 



draw. Under abnormal and well-nigh impossible 

 conditions, the organism, high or low, is an automa- 

 ton the creature of inevitable nervous responses- 

 reflex or conscious. Under normal conditions of life, 

 it responds now this way and now that to external 

 or internal stimuli and so appears to act as a free 

 agent. 



The apparent inevitability of reflexes is but an 

 indication of habit. When the environmental cir- 

 cumstances to which an organism is exposed are 

 comparatively simple or when the organism itself is 

 not highly differentiated, one or two external agents 

 may serve it as guides. The organism takes the habit, 

 for example, of relying implicitly on the stimuli of light 

 and gravity. By responding to these stimuli, it finds 

 its proper place with such certainty that other modes 

 of response to other stimuli are ignored habitually. 

 Hence, by playing on its habitual tropisms, it is 

 easy in the laboratory to lure an organism to its 

 doom. 



This we may illustrate by exposing C. roscoffensis 

 to simultaneous stimulation by light and heat. It 

 must be premised that the animal, though, for a 

 marine organism very tolerant of high temperatures, 

 is negatively therniotropic at about 35 C. At this 

 temperature it moves in the direction of the colder 

 water. In order to investigate the behaviour of the 

 animal with respect to simultaneously applied light- 



