12 SCIENCE AND PHILOSOPHY OF THE ORGANISM 



lation if the stimulus reaches a certain intensity, and 

 conversely. This is a rather simple complication, but an 

 additional phenomenon appears if the increased intensity of 

 the stimulus has lasted for some time. Then the organism 



o 



becomes adapted, or rather acclimatised, to this intensity, 

 and resumes the positive irritability it had before. 



Let us remember on this occasion what was said on 

 irritability and its restoration after irritation in the first 

 part of this work : all tropistic irritability follows the so- 

 called law of Weber, that is to say, an increase of the 

 intensity of the stimulus always acts only in proportion to 

 the intensity already present. This law resembles the so- 

 called " action of masses " in chemistry, and tends to prove 

 that something chemical is connected with tropisms. Also 

 the reversion of tropisms might be explained in the same 

 simple manner. But the change of the point of rever- 

 sion is another thing a real " acclimatisation," unknown 

 to us in its details, a real " secondary regulation," which, 

 though not proving vitalism in itself, is in any case very 

 remarkable. 



The second complication in the theory of tropisms 

 appears whenever the general conditions of life are altered. 

 In this case a change say of the general temperature of the 

 medium changes the " sense " of say heliotropism ; a fact 

 that has been named " heterogeneous induction ' by Noll. 

 This change of the sense of a tropism very often plays a 

 true morphogenetic, or, rather, restitutive role : if a pine is 

 decapitated, one of the side branches assumes the negative 

 geotropism of the lost main axis, and a similar phenomenon 

 holds for roots. The general organisatory state of the 

 organism is the " general condition ' that was altered in 



