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 



