50 MENTAL EVOLUTION IN ANIMALS. 



extremely doubtful whether any nerve in the human body, 

 even if in an inflamed condition, would be in any way 

 affected by such a particle supported in a dense fluid, and 

 slowly brought into contact with the nerve. Yet the cells 

 of the glands of Drosera are thus excited to transmit a 

 motor impulse to a distant point, inducing movement. It 

 appears to me that hardly any more remarkable fact than 

 this has been observed in the vegetable kingdom." 



But the case does not end here. For in another insec- 

 tivorous plant, Dionoea, or Venus' Fly-trap, the principle of 

 discriminating between different kinds of stimuli has been 

 developed in a direction exactly the opposite to that which 

 obtains in Drosera. For while Drosera depends for capturing 

 its prey on entangling the latter in a viscid secretion from its 

 glands, Dionoea closes upon its prey with the suddenness of 

 a spring-trap ; and in relation to this difference in the mode 

 of capturing prey, the principle of discrimination between 

 stimuli has been correspondingly modified. In Drosera, as we 

 have seen, it is the stimulus supplied by continuous pressure 

 that is so delicately perceived, while the stimulus supplied 

 by wijMct is disregarded ; but in Dionoea the smallest impact 

 upon the irritable surfaces, or filaments, is immediately re- 

 sponded to, while the stimulus supplied even by compara- 

 tively great pressure upon the same surfaces is wdioUy 

 disregarded. Or, in Mr. Darwin's own words, '' Although the 

 filaments are so sensitive to a momentary and delicate touch, 

 they are far less sensitive than tlie glands of Drosera to pro- 

 longed pressure. Several times I succeeded in placing on 

 the tip of a filament, by the aid of a needle moved with 

 extreme slowness, bits of rather thick human hair, and these 

 did not excite movement, although they were more than ten 

 times as long as those which caused the tentacles of Drosera 

 to bend ; and although in this latter case they were largely 

 supported by the dense secretion. On the other hand, the 

 glands of Drosera may be struck with a needle, or any hard 

 object, once, twice, or even thrice, with considerable force, 

 and no movement ensues. This singular difference in the 

 nature of the sensitiveness of the filaments of Dionoea and 

 of the glands of Drosera evidently stands in relation to the 

 habits of the two plants. If a minute insect alights with its 

 delicate feet on the glands of Drosera, it is caught by the 

 viscid secretion, and the slight, though prolonged pressure 



