ix ANIMAL BEHAVIOUR 165 



in part tropistic in relation to the magnetic currents of 

 the earth. 



A tropistic movement is the outcome of an attempt to 

 secure physiological equilibrium, and it goes on till that 

 is attained or till disaster ensues. They are obligatory 

 in the sense that every creature of the same kind and in 

 the same physiological state will in the same situation 

 behave in the same way. But it is a notable fact that a 

 tropism may be changed, reversed, or annulled by 

 changes in the physiological condition of the body or 

 by changes in the surrounding medium. There is ex- 

 perimental evidence that the tropistic movements may 

 be occasionally interrupted by individual internal initia- 

 tive on the part of the organism, may be inhibited or 

 departed from under the influence of some stronger in- 

 ternal stimulus. 



7. Trial Movements. We have given prominence to 

 reflex actions and tropisms because it seems certain that 

 many of the movements of the lower animals are of one 

 kind or the other. But it seems equally certain that 

 there are a good many " trial movements ' of a less 

 obligatory character. As Jennings says : " Unpreju- 

 diced observation of most Invertebrates will show that 

 they perform many movements which have no fixed 

 relation to sources of external stimuli, but which do serve 

 to test the surroundings and thus to guide the animal." 

 Holmes writes : 



" The lives of most insects, crustaceans, worms, and hosts of 

 lower Invertebrate forms, including even the Protozoa, show an 

 amount of busy exploration that in many cases far exceeds that 

 made by any higher animal. Throughout the animal kingdom 

 there is obedience to the Pauline injunction, ' Prove all things, hold 

 fast to that which is good.' 



Among simple multicellular animals there is not a 

 little of that restless locomotion which we referred to as 

 general organismal motor-activity in Infusorians. But 

 at any moment this may give place to controlled be- 

 haviour. The creature makes sensori-motor experiments 

 which work towards an end, such as the systematic 



