20 THEORY OF ORGANIC EVOLUTION. 



idioplasm reaches, after a time, a maximum, and thus 

 comes to an end, either because the nature of the 

 substance permits no new rearrangement or because 

 the stimulus is no longer active. The cessation of 

 the stimulus results from a micellar rearrangement 

 which indicates the character of the adaptation. If 

 the action of the stimulus lasts for only a short time, 

 the incipient rearrangement of the idioplasm stops, 

 or proceeds independently on account of the impulse 

 received, and the determinant becomes capable of 

 development, even after the impulse has long ceased 

 to act. 



Since various intervening transpositions follow 

 upon a stimulus in the organism, the final result 

 which appears as a reaction may turn out variously. 

 The same external causes may, according to the 

 nature of the organism and other circumstances, 

 have very unlike variations as a result. But the 

 internal rearrangement produces in a definite case 

 very definite variations. 



On account of the various intermediate steps it 

 is often difficult to discover the external cause of a 

 given adaptive variation. In many cases we recog- 

 nize it without difficulty in a definite mechanical 

 process or in warmth, light or evaporation. For 

 the most part the stimulus awakens in the organism 

 merely a want, which the reaction of the organism 

 endeavors to supply. Hence it appears that want 

 or lack alone is able to bring about such reactions. 



