54 THE METHOD OF DARWIN. 



for catching insects. To him adaptation for 

 insect catching meant that this habit was 

 advantageous to the plant; and that it must 

 derive nourishment from the captured insects. 

 He knew what elements plants required for 

 nourishment, and immediately set about mak- 

 ing another crucial test. When led to believe 

 that the leaves absorbed nutritious matter from 

 the insects, he made a crucial experiment by 

 immersing numbers of the leaves in nitro- 

 genous and non-nitrogenous fluidsW the same 

 specific gravity to find whether they would 

 act differently in the two cases, taking care 

 that the two sets of conditions should be the 

 same except in the presence of nitrogenous 

 matter in the one and its absence in the other. 

 The test confirmed his belief, and is an 

 example of the most rigid type of reasoning 

 and experiment, a combination of positive 

 and negative evidence which Mill called the 

 Method of Difference. Darwin's clear notion 

 of what constitutes good evidence led him to 

 seek demonstrative evidence by the shortest 

 way. In the case just described he secured it 

 at once, and might have rested content with 

 having established the principle; but as soon 

 as he found that the nitrogenous fluid alone 

 excited energetic movements " it was obvious 



