394 



SCIENCE 



[N. S. Vol. XXX. No. 769 



excitation from the surface to the deeper 

 parts, as well as from one part of the sur- 

 face to another. Prom this same layer 

 were produced the cells which, as muscle 

 fibers, would act as the motive mechanism 

 of the organism. Thus, from the begin- 

 ning, the chief means of attack or escape 

 were laid down in close connection with 

 the surface from which the stimuli were 

 received. A further step in the evolution 

 of the nervous system consisted in the 

 withdrawal of certain of the sensory or 

 receptor cells from the surface, so that a 

 specially irritable organ, the central nerv- 

 ous system, was evolved, which could 

 serve as a distributing center for the mes- 

 sages or calls to action initiated by changes 

 occurring at the surface of the body. At 

 its first appearance this central nervous 

 system would hardly deserve the epithet 

 of "central," since it formed a layer lying 

 some distance below the surface, and ex- 

 tending over a considerable area; though 

 we find that very soon there is an aggrega- 

 tion of the special cells to form ganglia, 

 each of which might be regarded as pre- 

 siding over the reactions of that part of the 

 animal in which it is situated. Thus in 

 the segmental worm-like animals a pair of 

 ganglia is present in each body segment, 

 and the chain of ganglia are united by 

 longitudinal strands of nerve fibers to form 

 the ganglionated cord, or central nervous 

 system. 



Such a diffused nervous system, in which 

 all ganglia were of equal value, could, how- 

 ever, only act for the common weal of the 

 whole body when a reaction initiated by 

 stimulation at one part was not counter- 

 acted by an opposing reaction excited from 

 another part of the surface. For survival 

 it is necessary that in the presence of 

 danger, i. e., an environment threatening 

 the life of the individual or race, the 

 whole activities of the organism should be 



concentrated on the one common purpose, 

 whether of escape or defense. This could 

 be effected only by making one part of the 

 central nervous system predominant over 

 all other parts, and the part which was 

 chosen for this predominance was the part 

 situated in the neighborhood of the mouth. 

 This, in animals which move about, is the 

 part which always precedes the rest of the 

 body, and therefore the part which first ex- 

 periences the sense impressions, favorable 

 or dangerous, arising from the environ- 

 ment. It is this end that has to appreciate 

 the presence or approach of food material, 

 as well as the nature of the medium into 

 which the animal is being driven by the 

 movements of its body. Thus a predom- 

 inance of the front end of the nervous 

 system was determined by the special de- 

 velopment at this end of those sense or- 

 gans or sensory cells which are proficient 

 — i. e., are stimulated by changes in the 

 environment proceeding from disturbances 

 at a distance from the animal. The sen- 

 sory organs of vision and the organs which 

 correspond to our olfactory sense organs 

 and are aroused by minute changes in 

 chemical composition of the surrounding 

 medium, are always found especially at 

 the front or mouth end of the organism. 

 The chances of an animal in the struggle 

 for existence are determined by the degree 

 to which the responses of the animal to the 

 immediate environment are held in check 

 in consequence of stimuli arising from 

 approaching events. The animal, without 

 power to see or smell or hear its enemy, 

 will receive no impulse to fly until it is 

 already within its enemy's jaws. It must 

 therefore be an advantage to any animal 

 that the whole of its nervous system 

 should be subservient to those ganglia or 

 central collections of nerve cells which are 

 in direct connection with the projicient 

 sense organs in the head. This subservi- 



