34 DEVELOPMENT AND HEREDITY. 



example, suppose the wire to be kept twisted through 

 90° to the right for six hours, then for half an hour 

 90° to the left, and then so gradually let go that 

 there is no oscillation. When it is left to itself, it 

 turns slowly toward the right, gradually undoing part 

 of the effect of the more recent twist, then stops, 

 and twists still more slowly to the left, thus undoing 

 part of the quasi-permanent effect of the earlier twist. 

 Thus the behaviour of such a wire, strictly speaking, 

 is an excessively complex one, depending, as it were, 

 upon its whole previous history ; though, of course, 

 the trace left by each stage of its treatment is less 

 marked, as the date of that stage is more remote." 



Professor J. Clerk Maxwell has expressed this last 

 idea in almost the same words, " the stress at any 

 given instant depends not only on the strain at that 

 instant, but on the previous history of the body." 



From all of this we may learn, first, how slight a 

 force may change the structure of a body ; second, 

 how complex may be its results ; and third, how 

 subtle and persistent may be the effects of a force 

 upon a body even though its action be only tempo- 

 rary. The nature of the changes which various 

 forces cause in bodies, will be more fully understood 

 by a careful consideration of the following account 

 of the constitution of bodies, quoted from Professor 

 Maxwell's article on that subject, in the British 

 Encyclopedia : — 



