1 76 THE METHOD OF DARWIN. 



centre of the earth. In accordance with these 

 conclusions, a considerable difficulty in the way 

 of evolution is in part removed, for it might 

 have been asked, How did all their diversified 

 movements for the most different purposes first 

 arise ? As the case stands, we know that there 

 is always movement in progress, and its ampli- 

 tude or direction, or both, have only to be mod- 

 ified for the good of the plant in relation with 

 internal or external stimuli. " 1 



Thus the great work of observation and rea- 

 soning began with an effort to explain the power 

 of climbing among plants under the theories of 

 descent and natural selection; passed on to the 

 prediction of the universal movement of cir- 

 cumnutation and its verification; and closed by 

 explaining all the other highly specialized and 

 remarkable movements 'of plants and plant or- 

 gans as modifications of the same general but 

 unapparent movement. The principal difficulty 

 at first was the fact that climbers were found 

 throughout the plant kingdom, and could not 

 have been descended from a common climbing 

 ancestor. By the investigations of Darwin and 

 his son, not only were the different methods of 

 climbing shown to be modifications of the twin- 

 ing movements of the stem, but it and all the 



1 Power of Movement in Plants, p. 4. 



