FACTS AND FACTORS OF DEVELOPMENT 51 



ment with that of other animals, but the great 

 principle of the oneness of life, as respects its 

 fundamental processes, has never yet failed 

 to hold true and will not fail us here. In the 

 study of the psychical processes of organisms 

 other than ourselves we are compelled to rely 

 upon a study of their activities, their reactions 

 to stimuli, since we can not approach the sub- 

 ject in any other way. The reactions and be- 

 havior of organisms under normal and experi- 

 mental conditions give the only insight which 

 we can get into their psychical processes; 

 and this applies to men no less than to 

 protozoa. 



1. Sensitivity. — The most fundamental 

 phenomenon in the behavior of organisms is 

 irritability or sensitivitj^, which is the ability 

 of receiving and responding to stimuli: this is 

 one of the fundamental properties of all proto- 

 plasm. But living matter is not equally sensi- 

 tive to all stimuli, nor to all strengths of the 

 same stimulus. Many of the simplest unicel- 

 lular plants and animals show that they are 

 differentially sensitive; they often move to- 

 ward weak light and away from strong light, 



