8o ESSAYS OF A BIOLOGIST 



Keyser,^ in a suggestive article, has characterized 

 this unique attribute of man by calling him * the 

 time-binder.' 



Speech and reasoning, with all their consequences, 

 have only been rendered possible through another 

 important qualitative change in the human brain, 

 which in its turn has led to other new potentialities 

 of life being realized in man and in man alone — its 

 flexibility. 



In some of the lowest forms of life, such as Para- 

 mecium, there are but one or two possible modes of 

 reaction — reactions which it attempts in response to 

 any one of the myriad changes that may occur in the 

 outer world. As we ascend the scale, we find two 

 chief types of alterations : in the first place an increase 

 in the number of hereditarily-given modes of reaction, 

 and in the second an increased power of * learning,' 

 of altering behaviour in adjustment to experience. 

 In the insects, the first is chiefly in evidence. Al- 

 though many insects undoubtedly can profit by 

 experience to a limited degree, yet most of their 

 behaviour is instinctive, in the sense that it unrolls 

 itself automatically and efliciently in the absence of 

 previous experience or of any possible instruction. In 

 the vertebrates, on the other hand, we see as we pass 

 from the lower to the higher groups a definite, steady 

 increase in the power of learning by experience, from 

 the fish that takes weeks to associate a given colour 

 1 Science y September 1921. 



