SENSITIVITY IN PLANTS 33 



complexity, and a similar setting aside of some part of 

 the brain to attend to the stimuli received. This com- 

 plexity was not reached all at once, and so the brain, 

 as we know it in man, must have passed through a very 

 long developmental history, constantly adding to its 

 already complex mechanism, and demanding also a 

 corresponding complexity in all the structures associated 

 with it. 



Enough has been said to show that in the plant and 

 the animal world alike there has been a division of labour 

 and consequently a differentiation of structure, of which 

 we can trace some steps imperfectly, it is true, and with 

 many gaps. Every step in this increasing complexity 

 has been fraught with momentous results for the higher 

 animals, including man. 



CHAPTER VI 



SENSITIVITY IN PLANTS AND ANIMALS 



THE power of responding to stimuli from within or 

 without is a universal and fundamental characteristic 

 of all living organisms ; and it forms the starting-point 

 of all manifestations of life. The property by which 

 the organism responds to stimulation is known as 

 irritability. 



The stimuli that form the exciting cause of the irri- 

 table reactions are, (1) intrinsic or regulating, and 

 (2) extrinsic or modifying. 



The inherited or regulating stimuli simply determine 

 that the living organism or substance shall pass through 



c 



