ADAPTATIONS 153 



greater affinity for the lime-base than the 

 latter, drives it off and the operation is at an 

 end. The train of circumstances looks back- 

 ward for its explanation. 



In the case of Arcella this is not so. It, 

 too, no doubt, produces gas, but not for the 

 mere pupose of producing it, nor for the pur- 

 pose of uniting two chemical substances with 

 one another. It forms gas because that is its 

 means of overcoming a problem in locomotion 

 which has presented itself. Hence the forma- 

 tion of the gas loks forward for its explanation, 

 in other words it is one which has a telic 

 explanation and looks towards an end and that 

 end a link in a chain of events, for the 

 Arcella makes the gas, no doubt, in the first 

 instance, in order that it may turn right side 

 up, but chiefly in order that it may resume 

 the even tenor of its ways which have been 

 temporarily interfered with by the accident 

 which laid it on its back. We may, if we 

 like, say that it is adapted to get right side 

 up again by this process, but if we do, we 

 must not suppose that we have then explained 

 all that requires explanation, and if we do use 

 that phrase we may remember that no one 

 ever talks about carbonate of lime being 

 " adapted " to make gypsum and consider 

 why the terminology which we think suit- 



