74 ' Luck, OR CUNNING? 



establish a modus vivendi with their surroundings. 

 They can do this because both they and the surround- 

 ings are plastic within certain undefined but somewhat 

 narrow limits. They are plastic because they can to 

 some extent change their habits, and changed habit, if 

 persisted in, involves corresponding change, however 

 slight, in the organs employed ; but their plasticity 

 depends in great measure upon their failure to per- 

 ceive that they are moulding themselves. If a change 

 is so great that they are seriously incommoded by its 

 novelty, they are not likely to acquiesce in it kindly 

 enough to grow to it, but they will make no diffi- 

 culty about the miracle involved in accommodating 

 themselves to a difference of only two or three per 

 cent.* 



As long as no change exceeds this percentage, and 

 as long, also, as fresh change does not supervene till 

 the preceding one is well established, there seems no 

 limit to the amount of modification which may be 

 accumulated in the course of generations — provided, 

 of course, always, that the modification continues to be 

 in conformity with the instinctive habits and physical 

 development of the organism in their collective capa- 

 city. Where the change is too great, or where an 

 organ has been modified cumulatively in some one 

 direction, until it has reached a development too 

 seriously out of harmony with the habits of the 

 organism taken collectively, then the organism holds 

 itself excused from further effort, throws up the whole 



* See Professor Hering's " Zur Lehre von der Beziehung zwisohen 

 Leib und Seele. Mittheilung Uber Feohner's psychophj'sisohes Gesetz.'' 



