ADAPTATIONS 95 



feebly moving its legs and wondering apparently 

 how it is ever going to get upon them again. 



Arcella has its own method of achieving this 

 task. It sets to work and by means of some 

 mechanism of which we know nothing, it produces 

 gas-bubbles in its interior. 



These are arranged in one of two ways. In 

 some cases they form at one side of the body only. 

 The result is that that side is floated upwards and 

 the little creature comes to stand on its edge. 

 From that position it is easy, by the aid of its 

 pseudopodia, for it to resume its normal position 

 with the pseudopodia downwards. 



In other cases the gas-bubbles, instead of being 

 confined to one part of the body, are generally dis- 

 seminated throughout it. The consequence of this 

 is that the entire organism becomes sufficiently 

 light to float to the top of the water in which it is 

 immersed. There it is able to assume the position 

 which it desires. 



Such a purposive action as this is wholly differ- 

 ent from the actions of chemical substances or of 

 those which we study in the physical laboratory. 



Forces of this latter kind look backward, not 

 forward, for their explanation. They are the re- 

 sult of what has been and the end of the series of 

 occurrences is an equilibrium. 



Let us take an example. Suppose we add oil 



