VI ANIMALS AND PLANTS 171 



Again, I know of no test by which the reaction 

 of the leaves of the Sundew and of other plants to 

 stimuli, so fully and carefully studied by Mr. 

 Darwin, can be distinguished from those acts of 

 contraction following upon stimuli, which are 

 called " reflex " in animals. 



On each lobe of the bilobed leaf of Venus's fly- 

 trap (Dioncea muscipula) are three delicate filaments 

 which stand out at right angle from the surface of 

 the leaf. Touch one of them with the end of a 

 fine human hair and the lobes of the leaf instantly 

 close together l in virtue of an act of contraction 

 of part of their substance, just as the body of 

 a snail contracts into its shell when one of its 

 " horns " is irritated. 



The reflex action of the snail is the result of the 

 presence of a nervous system in the animal. A 

 molecular change takes place in the nerve of the 

 tentacle, is propagated to the muscles by which the 

 body is retracted, and causing them to contract, 

 the act of retraction is brought about. Of course 

 the similarity of the acts does not necessarily in- 

 volve the conclusion that the mechanism by 

 which they are effected is the same ; but it 

 suggests a suspicion of their identity which needs 

 careful testing. 



The results of recent inquiries into the structure 

 of the nervous system of animals converge towards 

 the conclusion that the nerve fibres, which we 

 1 Darwin, Insectivormis Plants, p. 289. 



