The substance of this book was contained in outline in a series of Lectures, delivered, in accordance with the conditions of the Shaw Fellowship, at the University of Edinburgh, in January, 1889. Since that time, however, it has been very considerably altered and enlarged; and I have followed the example of my predecessor, Professor W. R. Sorley, in not retaining the Lecture form. AS the work is not meant to be used as a text-book, I have not thought it necessary to introduce minute subdivisions of chapters, but have preferred to leave it in the form of Six continuous essays.
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