THE ROOT-PPJNCIPLES OF MIND. 49 



water. That is to say, the tentacles are able to discrimi- 

 nate between the stimulus which is supplied by the turmoil 

 of the water and that which is supplied by their contact 

 with the solid body, and they respond to the latter stimulus 

 notwithstanding that it is of incomparably less intensity 

 than the former. And it is this power of discriminating 

 between stimuli, irrespective of their relative mechanical inten- \ 

 sities, that I regard as the objective principle of which we are 

 in search ; it constitutes the physiological aspect of Choice. 



A similar power of discriminative response has long been 

 known to occur in plants, though tlie most carefully observed 

 facts with regard to this interesting subject are those which 

 we owe to the later researches of llr. Darwin and his son. 

 The extraordinary delicacy of discrimination which these 

 researches show the leaves of plants to exercise between 

 darkness and light of the feeblest intensity, is not less 

 wonderful than the delicacy of discrimination which they 

 show the roots of plants to exercise in feeling about for 

 moisture and lines of least resistance in the soil. But in the 

 present connection the most suggestive facts are those which 

 have been brought to light by Mr. Darwin's previous re- 

 searches on the climbing and insectivorous plants. For, 

 from these researches it appears that the power of discrimi- 

 nating between stimuli, irrespective of relative mechanical 

 intensity or amount of mechanical disturbance, has here 

 proceeded to an extent that rivals the function of nerve- 

 tissue, although the tissues which manifest it have not in 

 structure passed beyond the cellular stage. Thus, the tenta- 

 cles of Drosera, which close around their prey like the 

 tentacles of a sea- anemone, will not respond to the violent 

 stimulation supplied by rain-drops falling upon their sensi- 

 tive surfaces or glands, while they will respond to an incon- 

 ceivably slight stimulus of the kind caused by an exceedingly 

 minute particle of solid matter exerting by gravity a con- 

 tinuous pressure upon the same surfaces. For Mr. Darwin 

 says, " The pressure exerted by a particle of hair, weighing 

 only -j^] 40 of a grain, and supported by a dense fluid, must 

 have been inconceivably slight. We may conjecture that it 

 could hardly have equalled the millionth of a grain ; and we 

 shall hereafter see that far less than the millionth of a grain 

 of phosphate of ammonia in solution, when absorbed by a 

 gland, acts on it and induces movement. ... It is 



D 



