-----
Expressing my sense of obligation to the authorities of the London School of Economics, where I have held the Shaw Research Studentship. Special thanks are due to Professor Edwin Cannan for valuable advice and criticism. To Mr. W. G. Constable and W. W. T. Layton I make grateful acknowledgments for fruitful suggestions and assistance. In revising the proof -sheets I have again received serviceable help from Mr. W. G. Constable, as also from my father, and from my brother, Mr. O. R. Hobson.
{{comment.content}}