CHAP, xi Sensitivity and Stimulation 461 



the behaviour in any case must be explained by the indi- 

 vidual organization or constitution. He thus first indicated 

 the direction that modern thought has taken with regard 

 to the irritability of plants and the part it plays in the 

 active life of the organism. By thus opposing the mechani- 

 cal theory held by most botanists of the early sixties, 

 under the leadership of Hofmeister, he did much to bring 

 back opinion to the original position of Dutrochet and 

 Treviranus and to prepare the minds of vegetable physio- 

 logists to accept the view of the purposeful character of 

 sensitivity. 



The researches and writings of Charles Darwin form 

 a very prominent feature of the literature of this time. 

 Apart from the great philosophic work on the Origin of 

 Species, the influence of which on biological science has 

 already been discussed, we find from 1875 to 1880 a con- 

 tinuous output of work on botanical subjects, to which may 

 in the main be attributed the new views the development 

 of which we are trying to trace. In all alike we find 

 sound reasoning, based upon careful and well-controlled 

 experiment, and combined with absolutely impartial criti- 

 cism of evidence whether derived from his own experiments 

 or from those of other workers. In the last of these 

 magnificent contributions to science, The Power of Movement 

 in Plants (1880), he had the assistance and co-operation 

 of his son, Francis Darwin, to whose subsequent researches 

 the physiology of sensitivity owes so much. In this book 

 we find definitely stated the idea partially led up to by Frank, 

 and suggested for heliotropism and geotropism by Sachs 

 in 1874 and 1875, that growth curvatures are phenomena 

 of irritability or sensitiveness. Darwin, however, went 

 further than this. In the course of his work he deter- 

 mined that the sensitive mechanism consists of two parts, 

 the one perceptive or sensory, the area of stimulation the 

 other responsive or motor, the area of movement or secre- 



