PREFACE. IX 



reasoning of this Essay, whole volumes 

 might be written without exhausting 

 w^hat is to be said upon them. I shall 

 be content, in the meantime, if this 

 slight sketch of so great a subject 

 should be of any use in directing others 

 into some well-defined paths of thought 

 and of investigation in regard to it. 



London, Dec, 9, 1868. 



