48 HEREDITY AND ENVIRONMENT 



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 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 



