12 INTRODUCTION 



or when the shape of a given member can be shown to be partially due 

 to pressures arising automatically during the process of development 1 . 

 In the case of both internal and external stimuli, it is possible to distinguish 

 between thermal, chemical, mechanical, photic, and electric stimuli, accord- 

 ing to the character of the operating agency. 



Manifestations of irritability are a fundamental property of the vital 

 mechanism, and there is perhaps no vital process which is uninfluenced 

 by stimuli, which permit, cause, restrict, or regulate, the particular action 

 in question. Irritability is indeed a general characteristic of all living 

 substance, and is exhibited by the lowest as well as the highest forms. 

 By the development of specific and special forms of irritability, a plant 

 is enabled to react in relation to the external world in the most various 

 ways. Particular manifestations of irritability are also essential in order 

 to regulate and connect the various parts of the vital mechanism, in 

 the developing organism as well as in the adult one. Thus the pro- 

 gress of any given vital activity continually gives rise to stimuli, now 

 accelerating, now inhibiting, the process in question, and so regulating 

 it ; while, under particular conditions, either the original activity may be 

 gradually modified, or new activities may be aroused. All phenomena 

 of correlation are brought about in this manner, that is to say, by the 

 operation of regulating stimuli reaching to all parts of the organism. 

 The regulatory mechanism of all complicated machines is similar in 

 character. Thus in the marine engine, the slide- and safety-valves, the 

 eccentrics and governors, the condensers and feed-pipes, are all automatic 

 in action, and the whole operates more economically and harmoniously 

 than if it were built up of a series of engines, one for each action, all 

 working together, but each one distinct from the others. 



Every result which follows stimulation is a manifestation of irrita- 

 bility ; it is immaterial from this point of view whether a noticeable 

 movement immediately occurs, or whether there results merely some obscure 

 chemical change, days or weeks after the stimulus has been applied. It 

 is only when perceptible results are produced that irritability can be 

 measured or determined. A moth, flying towards a candle, reacts to the 

 stimulus of light by movement in a particular direction, whereas an attached 

 plant responds by gradually bending towards the source of illumination. 

 In both cases the sign and result of the operation of the given stimulus 

 is the production of a definite movement. 



In the most varied and general physiological problems, questions con- 

 nected with irritability are constantly and continually arising. The clear 



1 It is incorrect to denote as the primary cause, either of a reaction, or of the liberation of 

 energy for that reaction, what may be really a single link in a chain of causes. According to 

 Clifford, Plato uses the word 'cause' in sixty-four different senses. See also Sigwart, Logic, 1895, 

 vol. ii. p. 93. 



