THE ORIGINS OF MIND IO9 



and respond in other ways to the needs of their metaboHsm. 

 The activities of man, his perception, and his thought are 

 the culmination of the movements of a protozoan which 

 responds to the warmth of the water, to chemical substances 

 dissolved in the water, to hard objects in its path, to the 

 stimulus of food, to the presence of its enemies. There is 

 now good evidence that this lowly, microscopic organism 

 can even learn from experience and modify its behavior 

 accordingly. This creature and all other creatures act as 

 they do because of three rudimentary properties of be- 

 havior: first, receptivity, the influence of the external en- 

 vironment upon the organism; second, response, the activity 

 of the organism; and third, correlation, actions brought into 

 relationship with circumstances, at first automatically and 

 later in conscious and deliberate thought processes. 



Receptivity is the result of a very fundamental property 

 of protoplasm, namely, irritability. It is a universal property 

 of protoplasm, and without it there could be no nervous 

 activity, no sensation, or consciousness. At first the property 

 is very generalized and without organs of perception, but 

 gradually it becomes more and more specialized and efficient 

 through the evolution of sense structures. Light is one of 

 the many phenomena in nature to which protoplasm is ir- 

 ritable. If the naked protoplasm of an amoeba is treated with 

 a beam of intense light, the animal first contracts the local 

 region touched by the light and then the rest of the body, 

 and moves as rapidly as possible into the shadow. There is 

 no organ of sight in the amoeba, but nevertheless it is sensi- 

 tive to variations in the light striking its habitat. Some uni- 

 cellular organisms, like amoeba, move away from strong 

 light, others are attracted to it. All have a degree of sensi- 

 tivity. 



From this generalized irritability to light, nature has 

 evolved many and varied receptors of which the human eye 

 is one. In some protozoa there is a pigment spot near the 

 forward end that is especially sensitive to light. No image is 



