GREAT STEPS IN EVOLUTION 93 



of stimulus, only one answer to every ques- 

 tion, but there are others whose behaviour 

 is at a higher level, illustrating what may be 

 called " the method of trial and error." They 

 "try" one kind of reaction after another, 

 until, in some cases, they give the effective 

 response. 



But while we cannot doubt that the be- 

 ginnings of behaviour are to be found in the 

 Protozoa, new possibilities must have opened 

 up whenever head-brains were established. 

 For this centralizing of the nervous system 

 must have meant a new integration, a more 

 unified control, of the whole organism. 



We cannot attempt to summarize the 

 discernible steps in the evolution of beha- 

 viour, but we wish to press home the fact that 

 what we are so familiar with to-day is the 

 long result of time. We see behaviour rising, 

 along one line, to its wonderful instinctive 

 expressions. We see it rising (whether fur- 

 ther or along another line is still under dis- 

 cussion) into intelligent expression where 

 there is perceptual inference. Finally, in 

 man, with his conceptual inferences, intel- 

 ligent behaviour becomes rational conduct. 



Progress along many Lines. — In his 

 interesting "Evolution of Plants," Dr. Scott 

 refers to the important fact that at a time so 

 remote as the Devonian period, when there 



