Circumnutatlon 399 



swims into a region where the temperature is too high or where an 

 injurious substance is present, it changes its course. It then moves 

 forward again, and if it is fortunate enough to escape the influence, 

 it continues to swim in the given direction. If however its change 

 of direction leads it further into the heated or poisonous region it 

 repeats the movement until it emerges from its difficulties. Jennings 

 finds in the movements of the lower organisms an analogue with 

 what is knowTi as pain in conscious organisms. There is certainly 

 this much resemblance that a number of quite different sub-injurious 

 agencies produce in the lower organisms a form of reaction by the 

 help of M'hich they, in a partly fortuitous way, escape from the 

 threatening element in their environment. The higher animals are 

 stimulated in a parallel manner to vague and originally purposeless 

 movements, one of which removes the discomfort under which 

 they suflfer, and the organism finally learns to perform the appro- 

 priate movement without going through the tentative series of 

 actions. 



I am tempted to recognise in circumnutation a similar ground- 

 work of tentative movements out of which the adaptive ones were 

 originally selected by a process rudely representative of learning by 

 experience. 



It is, however, simpler to confine ourselves to the assumption that 

 those plants have survived which have acquired through unknown 

 causes the power of reacting in appropriate ways to the external 

 stimuli of light, gravity, etc. It is quite possible to conceive this 

 occurring in plants which have no power of circumnutating — and, as 

 already pointed out, physiologists do as a fact neglect circumnutation 

 as a factor in the evolution of movements. Whatever may be 

 the fate of Darwin's theory of circumnutation there is no doubt 

 that the research he carried out in support of, and by the light 

 of, this hypothesis has had a powerful influence in guiding the 

 modern theories of the behaviour of plants. Pfeffer^, who more than 

 any one man has impressed on the world a rational view of the 

 reactions of plants, has acknoAvledged in generous words the great 

 value of Darwin's Avork in the same direction. The older view Mas 

 that, for instance, curvature towards the light is the direct mechanical 

 result of the difierence of illumination on the lighted and shaded 

 surfaces of the plant. This has been proved to be an incorrect ex- 

 planation of the fact, and Darwin by his work on the transmission 

 of stimuli has greatly contributed to the current belief that stimuli 

 act indirectlv. Thus we now believe that in a root and a stem the 

 mechanism for the perception of gravitation is identical, but the 

 resulting movements are diflierent because the motor-irritabilities 



1 The Fhyiiology oj Plants, Eng. Tr. iii. p, 11. 



