48 HEREDITY AND ENVIRONMENT 



we can get into their psychical processes; 

 and this apphes to men no less than to 

 protozoa. 



1. Sensitivity. — The most fundamental 

 phenomenon in the behavior of organisms is 

 irritability or sensitivity, which is the capacity 

 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, 

 away from extremes of heat and cold, into 

 certain chemical substances and away from 

 others; in short, all organisms, even the sim- 

 plest, may respond differently to different 

 kinds of stimuli or to different degrees of the 

 same stimulus. This is what is known as dif- 

 ferential sensitivity (Figs. 17, 18, 19) . On the 

 other hand, many organisms respond in the 

 same way to different stimuli, and this may be 

 taken to indicate generally that they are not 

 differentially sensitive to such stimuli; it is 



