In arranging the essays and lectures in this book for publication, their respective dates of production have been disregarded for the sake of presenting them, as far as possible, in a natural sequence. The first of the articles contains a statement of the fundamental principles of the author's hypothesis, and may be regarded as introductory to those which follow. It will be found that these principles are restated, briefly, in other parts of the book ; but the repetition is due to the fact that the papers were written at considerable intervals of time, and for readers or audiences not always familiar with the author's theories. I have ventured, however, to eliminate much of the iterated matter, leaving only that which bears directly upon the subject under consideration in the article in which it occurs.