284 THE SCIENCE OF LIFE. 



In the general account of the nervous system (Chap. 

 XV., Sec. i) it was stated that many motions were merely 

 reflex and involuntary. Many such motions are also 

 without consciousness. It is probable that a very large 

 proportion of the movements of the lower animals are 

 of this character. Other motions depend on organic 

 contractility responsive to an external stimulus, as when 

 a piece of muscular fiber contracts on being scratched 

 with a pin. Ciliary motions, the closure of the leaves of 

 Venus'-flytrap (Dionaa) on being touched by an insect, 

 and the movements of the Sensitive plant, may thus be 

 accounted for. Some motions, as the sleep of plants, 

 depend on the periodicity of functional activities, and 

 others, as the bursting of seed-vessels, may be owing to 

 Endosmose. Mere movement, therefore, is far from in- 

 dicating consciousness. 



" How early does consciousness arise ? If we inter- 

 pret, as we are constantly doing, the experience of lower 

 animals by that of higher ones, we should answer, With 

 the very commencement of animal life. Indeed, noth- 

 ing but conventional sentiment would prevent our attrib- 

 uting, under this method, a feeble consciousness to some 

 plants. If, however, we reason from the character of the 

 nervous system, which is undoubtedly the sole organ of 

 consciousness, and from the stages in development at 

 which a conscious experience can enter as a profitable 

 factor, we shall be inclined to believe that consciousness 

 especially characterizes the Vertebrata, and appears first 

 in the higher Articulata and Mollusca. The phenomena 

 of consciousness undoubtedly increase greatly in vigor 

 and in value as we pass up through the Vertebrata, and 



