34: GENERAL EVOLUTION 



scionsness. But how is consciousness aroused in the cases which 

 we can investigate — that is, in our own species ? In the common 

 case of sleep, a mere sound is sufficient to cause its rerurn. In 

 syncope and other forms of loss of consciousness, a sudden shock, 

 as of cold water thrown on the face, will often restore it. In 

 ordinary states, compulsion or pressure will produce a more in- 

 tense degree of consciousness. Indeed, in many persons conscious- 

 ness varies greatly under different influences ; thus a dreamy state 

 naturally follows a lack of nutrition of the brain, as during a fast, 

 but the mind is readily aroused from it by a strong or sudden im- 

 pression made on the senses or by an effort of will. It is, hence, 

 to be believed that in animals whose acts may be largely automatic 

 consciousness is aroused by exceptional sights, sounds, hunger, 

 and other sensible impressions, the more readily in proportion to 

 the development of the sensory ganglia of the brain. In the low- 

 est animals, consciousness will be aroused with much greater diffi- 

 culty, and life must be in part a blank, and in part a dream, at 

 rare intervals illumined by gleams of consciousness produced un- 

 der the influence of strong external stimulus. At these times of 

 awakening we must believe that new movements are instituted, 

 which will become habitual in proportion to the extent to which 

 they are repeated. 



From what is known of reflex actions, it is evident that move- 

 ments may become habitual, and hence automatic, wherever cellu- 

 lar or gray nerve-substance exists. Thus, movements instigated by 

 the spinal nerves derive their automatism from the spinal medul- 

 la. This axis no doubt derived the original nerve-force from the 

 brain, but repetition of the act under the original stimulus con- 

 fers the automatic power on the spinal cord or ganglionic center 

 as certainly as on the brain. This must of necessity follow the 

 complication of nervous structures by addition to the original 

 center. Thus are explained the automatic movements of the 

 frog's or centipede's legs in the well-known experiments, when 

 separated from the brain or principal nervous center. 



Greater difficulties are experienced in accounting for the origin 

 of the automatic movements of plants. It is evident, however, 

 that many of these, perhaps all, are not due to the energy of mo- 

 tive-force, but are phenomena of growth-force alone. Motion is 

 produced by the change of direction imparted to growth-force by 

 the influence of heat, light, contact, etc. This will largely follow 

 as a consequence of the longer persistence of growth in plants 



