LETTERS TO BROTHER JOHN. 221 



ports another, and is itself by others supported. 

 This is the invariable conduct of Nature. If 

 she had to prevent two houses from falling, she 

 would not get a prop for this and a prop for 

 that ; no ; she would make one house prop up the 

 other. 



It is this propping and dovetailing of one system 

 with another which constitutes what I mean by the 

 relation of one system to another ; as, for instance, 

 our own system to that of the world ; and which 

 makes it so impossible to destroy, or in any way 

 interfere with, that relation or adaptation, without 

 mischief to the individual system ; which is thus, as 

 it were, withdrawn from the support of the rest, 

 and without injury to the beauty and harmony of 

 the whole. 



Now, the things which we are accustomed to 

 consider " discomforts " are the very contrivances 

 by means of which our system is interwoven, as it 

 were, with and into the system of the world which 

 we inhabit, and are absolutely necessary to secure 

 this connexion. I will explain this. 



You will, I hope, remember, that there are four 

 conditions absolutely necessary to the existence of 

 all living things ; viz. organism, contractility, sensi- 

 bility, and STIMULI. The principal of these stimuli 



