24 MAN, THE ANIMAL 



ology, growth changes and the accompanying 

 differentiation of structure on the immediate re- 

 sult of unequal division in the biological unit. 

 Little is to be gained by trying to use the terms 

 development, growth, and differentiation, with a 

 specific meaning, for none of them aims to do more 

 than describe what happens after unequal division 

 in cells has taken place in the embryo and until 

 the adult form is reached. 



8. The Law of Sensation (Sensitivity or Ir- 

 ritability). — This is a phase of organic beings by 

 which they become aware of their environment or 

 make some change in their behavior because of it. 

 The general term describes several distinct proc- 

 esses, such as the power to appreciate an appro- 

 priate stimulus, to conduct the stimulus to a 

 recording center or centers, and to transform the 

 stimulus so that movement results. Just how many 

 distinct processes are involved in this most elusive 

 of all vital phenomena, we do not know. We are 

 unable to explain how the stimulus travels in 

 protoplasm, nor just what transforms the stimulus 

 so that a definite movement takes place. In higher 

 animals sense organs and a highly developed nerv- 

 ous system enable them to gain accurate informa- 

 tion concerning their surroundings, and for man 

 as well as for these higher animals this is the sole 

 avenue of information concerning material sub- 

 stances. Through this property come all of the 



