48 MAN AN ADAPTIVE MECHANISM 



that a central nervous system is not necessary for the 

 production of coordinated reflexes in an animal, then, 

 it was argued, it should be possible to obtain the same 

 coordinated reflex in an animal whose central nervous ' 

 system had been removed, provided a continuous path- 

 way of protoplasm could be maintained for the con- 

 duction of the exciting impulse from the skin to the 

 muscles. To prove this poinl- the central nervous 

 system was excised from earthworms and from ascid- 

 ians (Loeb). Excision of the central nervous system 

 in these animals did not destroy their characteristic re- 

 flexes, but the crux of the situation was disclosed in the 

 fact that, although the response of the mutilated animal 

 to stimulation was the same as that of the normal 

 animal, it required more stimulation to produce response 

 in the former, and the response was much more retarded 

 than in the normal animal. In other words the thresh- 

 old of stimulation had been raised by the destruction 

 of the specialized pathway over which the exciting 

 impulse was accustomed to travel. The central 

 nervous system was thus shown to be a quick and 

 sensitive conductor by means of which the efficiency 

 of the whole mechanism was increased. The evolu- 

 tion of specialized tissue is responsible for the attain- 

 ment of efficiency, for as we ascend the animal scale 

 we find that pari passu with the increasing differ- 

 entiation and development of certain tissues, the 

 environment dominated by the organism is widened. 



Thus as we progress from the simple protozoon . 

 to the complex pluricellular organism we find evolved 

 progressively more effectual organs of locomotion and 

 special sense ; more complicated systems of nutrition, 



