Rev. P. Keith on the Susceptibilities of Living Structures. 251 



even in animals, which evidently possess sensation, there are 

 still many movements of which the individual is not conscious. 

 Take the case of the circulation of the blood, or of the processes 

 of digestion, absorption, and nutrition, and you have a good 

 example of the fact. Besides, the action of light and of heat 

 would amount to something, even upon bodies destitute both 

 of life and of feeling. Some it might cause to expand, others 

 to contract, and others it might affect in a different way. — 

 Expose a piece of dyed cloth to the action of the direct rays 

 of the sun, and you will soon see its colour begin to fade. 

 Hold a sheet of paper, or of parchment before a brisk fire, 

 and it will immediately roll itself up into a scroll. Place a 

 snow-ball in the same situation, and it will quickly dissolve 

 into water. Try the experiment upon a piece of wet clay, 

 and it will shrink in all its dimensions. Repeat the experiment 

 upon a lump of wax, and it will speedily expand in volume. — 

 Hence we regard the bending of the stem towards the small 

 aperture in the dark room, and the unfolding of the flower at 

 a given temperature, as being nothing more than the chemical 

 action of light, or of heat attracting or expanding the vegetable 

 fabric, and thus facilitating the growth and nutrition of the 

 plant *. But vegetables, says Darwin, perceive not only the 

 impressions of stimuli, they perceive also negatives, and are 

 consequently endowed with sensation. — Cold and darkness are 

 negatives f. — I am not quite sure that chemists have decided 

 whether cold is a negative or an affirmative %. Darkness we 

 may regard as a negative. But if we regard them both in 

 that light, or if we regard them both in a contrary light, what 

 do we gain by it after all? — If the stimulus of heat or of light 

 is withdrawn, its effect must necessarily subside, whether its 

 withdrawal is perceived by the body that has been stimulated 

 or not. If the paper or parchment is withdrawn from before 

 the fire, and placed in a cooler atmosphere, it gradually re- 

 covers its former shape without any perception of the nega- 

 tion of cold. So also the flower closes, and the leaves fold up 

 without being conscious in any degree of the cause of their 

 own movements. 



If we do not concede the attribute of sensation to the phse- 

 nomenon of the excitability of vegetables by the action of the 

 natural stimuli of light and of heat, it has been thought that 

 we must at least concede it to the phaenomenon of their irrita- 

 bility by the action of accidental and extraneous or artificial 



♦ An excellent view of the subject, liarnionizinc; with that taken by Mr. 

 Keith, will be found in Sir H. Davy's Agricultural Chemistry, Lect. V. — Edit. 

 t Zoon. vol. i. Sect. 1.3. 

 t Surely it lias been decided that cold is a negative.— Edit. 



2 K 2 stimuli. 



