52 HEREDITY AND ENVIRONMENT 



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. 15-19). 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 



a B C D E b J? g 



Fig. 15. Distribution of Bacteria in the Spectrum. The 

 largest group is in the ultra-red at the left; the next largest 

 group is in the yellow-orange close to the line D. (From Jen- 

 nings, after Engelmann. - ) 



not to be concluded because organisms respond 

 differently to certain stimuli that they are 

 therefore capable of distinguishing between 

 all kinds of stimuli, for this is certainly not 

 true. Even in adult men the capacity of dis- 



