LONG PERSISTING ADJUSTMENTS 175 



doubt there is always some, lasting impression, for even the bar 

 of iron is never quite the same after it has been once struck ; 

 but the results of the slight organic changes we have been 

 alluding to are usually lost as the sand-ripples are lost when 

 the tide turns. They are the merely transient results of re- 

 sponses to frequently recurring environmental changes to which 

 the organism is well accustomed. 



3. Adjustments which persist for a Considerable Time.— 

 Insensibly, however — for it is all a matter of degree — we pass 

 from transient results to others which last for a considerable 

 time. We are browned by the sun on our summer holiday, and 

 the result may last far into the autumn. The change, though 

 still very superficial, has taken a firmer hold. The world is 

 full of illustrations — the increase in the child's weight after 

 a month at the farm, the increase in the size of the muscles after 

 a course of Sandow exercises, the warping of the plant-stem 

 which has been illumined from one side only, the blanching of 

 the banked-up celery. But these results do not last long after 

 the inducing conditions have ceased to operate. Sooner or 

 later there is a return to the normal. Like a bow unstrung, 

 the organism rebounds approximately to its previous state. 

 The stimulus ceases or the absent stimulus is restored, and 

 the organism, as if at the command " As you were," returns 

 to the status quo. 



4. Modifications. — Insensibly, however — for it is still only a 

 matter of degree — we pass from these temporary changes to 

 others which are demonstrably permanent. For there are cases 

 where the new stimulus provokes a structural change, which 

 persists after the stimulus has ceased. As we have put it, 

 metaphorically, the limit of organic elasticity has been trans- 

 cended. These are what in technical language we call " acquired 

 characters " or " modifications." 



The Englishman who works half his lifetime under a tropical 

 sun may become so tanned that the result does not disappear 



