September 3, 1908J 



NA TURE 



417 



This would be a gigantic task, for which I am quite un- 

 fitted. It seems to me, moreover, that the first duty of 

 your President is to speali on matters to wliich his own 

 researches have contributed. My wortc — such as it is — 

 deals with the movements of plants, and it is with this 

 subject that I shall begin. I want to give you a general 

 idea of how the changes going on in the environment act 

 as stimuli and compel plants to execute certain move- 

 ments. Then I shall show that what is true of those 

 temporary changes of shape we describe as movements is 

 also true of the permanent alterations known as morpho- 

 logical. 



I shall insist that, if the study of movement includes the 

 problem of stimulus and reaction, morphological change 

 must be investigated from the same point of view. In 

 fact, that these two departments of inquiry must be classed 

 together, and this, as we shall see, has some important 

 results — namely, that the dim beginnings of habit or un- 

 conscious memory that we find in the movements of plants 

 and animals must find a place in morphology ; and inas- 

 much as a striking instance of correlated morphological 

 changes is to be found in the development of the adult 

 from the o™m, I shall take this ontogenetic series and 

 attempt to show you that here also something equivalent 

 to memory or habit reigns. 



Many attempts have been made to connect in this way 

 the phenomena of memory and inheritance, and I shall 

 ask you to listen to one more such attempt, even though 

 I am forced to appear as a champion of what some of 

 you consider a lost cause — the doctrine of the inheritance 

 of acquired characters. 



In his book on " The Power of Movement in Plants " 

 (1S80) ' my father wrote that " it is impossible not to be 

 struck with the resemblance between the foregoing move- 

 ments of plants and many of the actions performed un- 

 consciously by the lower animals." In the previous year 

 Sachs ^ had in like manner directed attention to the essen- 

 tial resemblance between the irritability of plants and 

 animals. I give these statements first because of their 

 simplicity and directness ; but it must not be forgotten 

 that before this Pfeffer ' had begun to lay down the prin- 

 ciples of what is now known as Reizphysiologie, or the 

 physiology of stimulus, for which he and his pupils have 

 done so much. 



The words of Darwin which I have quoted afford an 

 example of the way in which science returns to the obvious. 

 Here we find revived, in a rational form, the point of view 

 of the child or of the writer of fairy stories. We do not 

 go so far as the child; we know that flowers do not talk 

 or walk ; but the fact that plants must be classed with 

 animals as regards their manner of reaction to stimuli 

 has now become almost a commonplace of physiology. 

 And inasmuch as we ourselves are animals, this concep- 

 tion gives us a certain insight into the reactions of plants 

 which we should not otherwise possess. This is, I allow, 

 a very dangerous tendency, leading to anthropomorphism, 

 one of the seven deadly sins of science. Nevertheless, it 

 is one that must be used unless the great mass of know- 

 ledge accumulated by psychologists is to be forbidden 

 ground to the physiologist. 



Jennings * has admirably expressed the point of view 

 from which we ought to deal with the behaviour of the 

 simpler organisms. He points out that vi'e must study 

 their movements in a strictly objective manner : that the 

 same point of view must be applied to man, and that any 

 resemblances between the two sets of phenomena are not 

 only an allowable but a necessary aid to research. 



What, then, are the essential characters of stimuli and 

 of the reactions which they call forth in living organisms? 

 Pfeffer has stated this in the most objective way. An 

 organism is a machine which can be set going by touch- 

 ing a spring or trigger of some kind; a machine in which 

 energy can be set free by some kind of releasing 

 mechanism. Here we have a model of at least some of 

 the features of reaction to stimulation. 



] P. 571.. 2 Arlcilen, ii. i?79, p. 282. 



•' Osttiotische Untersitchttitgen, 1877, p. 202. 



** " The Behaviour of the Lower Organisms," 1904, p. 124. 



NO. 2027, VOL. 78] 



The energy of tlie cause is generally out of all propor- 

 tion to the effect, i.e., a small stimulus produces a big 

 reaction. The specific character of the result depends on 

 the structure of the machine rather than on the character 

 of the stimulus. The trigger of a gun may be pulled in 

 a variety of different ways without affecting the character 

 of the explosion. Just in the same way a plant may be 

 made to curve by altering its angle to the vertical, by 

 lateral illumination, by chemical agency, and so forth ; the 

 curvature is of the same nature in all cases, the release- 

 action differs. One of those chains of wooden bricks in 

 which each knocks over the next may be set in action by 

 a touch, by throwing a ball, by an erring dog, in short 

 by anything that upsets the equilibrium of brick No. i ; 

 but the really important part of the game, the way in 

 which the wave of falling bricks passes like a prairie fire 

 round a group of Noah's Ark animals, or by a bridge over 

 its own dead body and returns to the starting-point, &c. — 

 these are the result of the magnificent structure of the 

 thing as a whole, and the upset of brick No. i seems a 

 small thing in comparison. 



P'or myself I see no reason why the term stiniiihts should 

 not be used in relation to the action of mechanisms in 

 general ; but by a convention which it is well to respect, 

 stimulation is confined to the protoplasmic machinery of 

 living organisms. 



The want of proportion between the stimulus and the 

 reply, or, as it has been expressed, the unexpectedness of 

 the result of a given stimulus, is a striking feature in the 

 phenomena of reaction. That this should be so need not 

 surprise us. We can, as a rule, only know the stimulus 

 and the response, while the intermediate processes of tlie 

 mechanism are hidden in the secret life of protoplasm. 

 We might, however, have guessed that big changes would 

 result from small stimuli, since it is clear that the success 

 of an organism in the world must depend partly at least 

 on its being highly sensitive to changes in its surround- 

 ings. This is the adaptive side of the fundamental fact 

 that living protoplasm is a highly unstable body. Here 

 I may say one word about the adaptation as treated in 

 the " Origin of Species." It is the present fashion to 

 minimise or deny altogether the importance of natural 

 selection. I do not propose to enter into this subject ; I 

 am convinced that the inherent strength of the doctrine 

 will insure its final victory over the present anti-Darwinian 

 stream of criticism. From the Darwinian point of vievi' 

 it would be a remarkable fact if the reactions of organisms 

 to natural stimuli were not adaptive. That they should be 

 so, as they undoubtedly are, is not surprising. But just 

 now I only direct attention to the adaptive character of 

 reactions from a descriptive point of view. 



Hitherto I have implied the existence of a general 

 character in stimulation without actually naming it ; I 

 mean the indirectness of the result. This is the point of 

 view of Dutrochet, who In 1824 said that the environment 

 suggests but does not directly cause the reaction. It is 

 not easy to make clear in a few words the conception of 

 indirectness. Pfeffer ' employs the word induction, anJ 

 holds that external stimuli act by producing internal 

 change, such changes being the link between stimulus and 

 reaction. It may seem, at first sight, that we do not 

 gain much by this supposition ; but since these changes 

 may be more or less enduring, we gain at least the con- 

 ception of after effect as a quality of stimulation. What 

 are known as spontaneous actions must be considered as 

 due to internal changes of unknown origin. 



It may be said that in speaking of the " indirectness " 

 of the response to stimuli we are merely expressing in 

 other words the conception of release-action ; that the 

 explosion of a machine is an indirect reply to the touch on 

 the trigger. This is doubtless true, but we possibly lose 

 something if we attempt to compress the whole problem 

 into the truism that the organism behaves as it does because 

 it has a certain structure. The quality of indirectness is 

 far more characteristic of an organism than of a machine, 

 and to keep it in mind is more illuminating than a slavish 

 adherence to the analogy of a machine. The reaction of 

 an organism depends on its past history ; but, it may be 



1 *' Physiology," Engl, edit., i. p. 11. 



