76 The Outline of Science 



the surrounding world; the living creature is acted on by influ- 

 ences from without. The organism acts on its environment; that 

 is the one side of the shield: the environment acts upon the 

 organism: that is the other side. If we are to see life whole we 

 must recognise these two sides of what we call living, and it is 

 missing an important part of the history of animal life if we 

 fail to see that evolution implies becoming more advantageously 

 sensitive to the environment, making more of its influences, 

 shutting out profitless stimuli, and opening more gateways to 

 knowledge. The bird's world is a larger and finer world than an 

 earthworm's; the world means more to the bird than to the worm. 



The Trial and Error Method 



Simple creatures act with a certain degree of spontaneity on 

 their environment, and they likewise react effectively to sur- 

 rounding stimuli. Animals come to have definite "answers back," 

 sometimes several, sometimes only one, as in the case of the 

 Slipper Animalcule, which reverses its cilia when it comes within 

 the sphere of some disturbing influence, retreats, and, turning 

 upon itself tentatively, sets off again in the same general direc- 

 tion as before, but at an angle to the previous line. If it misses 

 the disturbing influence, well and good ; if it strikes it again, the 

 tactics are repeated until a satisfactory way out is discovered or 

 the stimulation proves fatal. 



It may be said that the Slipper Animalcule has but one 

 answer to every question, but there are many Protozoa which have 

 several enregistered reactions. When there are alternative re- 

 actions which are tried one after another, the animal is pursuing 

 what is called the trial-and-error method, and a higher note is 

 struck. 



There is an endeavour after satisfaction, and a trial of 

 answers. When the creature profits by experience to the extent 

 of giving the right answer first, there is the beginning of 

 learning. 



