Circumnutation 899 
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 known 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 which 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 suffer, 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 acknowledged in generous words the great 
value of Darwin’s work in the same direction. The older view was 
that, for instance, curvature towards the light is the direct mechanical 
result of the difference 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 indirectly. 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 different because the motor-irritabilities 
1 The Physiology of Plants, Eng, Tr. mt. p. 11, 
