The Reversal of Tropisms 115 



forms of stimulation. There is much to indicate that 

 inhibition is intimately related to reversal of tro- 

 pisms in many forms, but I shall not venture upon 

 any speculations, which at best could only be very 

 tentative, as to its method of operation. 



It is quite possible that the explanation of reversal 

 of tropisms may be quite different in different cases. 

 Many rhizopods show a positive thigmotaxis to a 

 weak mechanical stimulus, while they give a negative 

 reaction if the stimulus is strong. Is this to be ex- 

 plained in the same way as the reversal in the beat 

 of the flagellum in a flagellate, or of the cilia in an 

 infusorian? And does any of these cases have any- 

 thing in common with the reversal of tropisms in 

 an insect or worm? The mechanism of orienta- 

 tion is quite different in different organisms, and it 

 seems not improbable that the inner mechanism of 

 reversal may be very different also, although there 

 may be broad underlying features common to nu- 

 merous apparently different cases. 



Up to the present time most of the work on the 

 reversal of tropisms has been done with a view of 

 ascertaining the various conditions under which re- 

 versals may occur. Many interesting facts have 

 been accumulated, but, as we have seen, they afford 

 no very sound basis for generalization. We shall 

 make little further progress by mere induction. We 

 need to feel our way along by making hypotheses 

 and testing them by appropriate experiments. But 

 when one attempts to attack the problem in this way 



