82 ESSAYS OF A BIOLOGIST 



it. Professor 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 in- 

 crease 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 evi- 

 dence. Although 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 efficiently in the ab- 

 sence 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 as- 

 sociate a given colour with a given event such as 



^Science, September 1921. 



