THE WAYS OF LIFE 199 



This happened four times without escape from the warmth. 

 Eventually, however, after these trials, a way of escape was 

 found. There are some who do not accept the interpreta- 

 tion that Jennings puts upon his facts ; but every one 

 admits that his facts are very important and that his 

 interpretation must be given a fair hearing. 



Professor Jennings has shown that the Protozoon 

 Stentor, a relatively large Infusorian, reacts to a precipita- 

 tion of powder in the water (1) by turning aside ; or if that 

 fails, by (2) reversing ; or if that fails, by (3) contracting 

 into its tube ; or if the precipitation continues, by (4) shift- 

 ing its quarters altogether. So far, trial of different 

 reactions, three of which were ineffective, though in other 

 circumstances any one might have been a perfectly good 

 answer. But the point is that when Professor Jennings, 

 after a short interval, repeated the fall of powder, the 

 Stentor began with the fourth answer. It had learned 

 something from its experience. 



It remains, in part, a matter of opinion, but to us it 

 seems impossible to describe the behaviour of Protozoa 

 as merely due to tropisms. What they do is not always 

 predictable, they seem to try different reactions, they seem 

 to learn from experience, they show discrimination or selec- 

 tion in what they pursue and in what they avoid. Professor 

 Jennings goes the length of saying : ' In no other group 

 of organisms does the method of trial and error so com- 

 pletely dominate behaviour, perhaps, as in the Infusoria '. 



THE STUDY or ANIMAL INSTINCT 



There are few problems that have been more discussed 

 than that presented by the instinctive behaviour of animals, 



