Excerpt from the Preface: "As in many other of these Studies, and perhaps more than in most, the task attempted in the present volume is mainly of a tentative and preliminary character. There is here little scope yet for the presentation of definite scientific results. However it may be in the physical universe, in the cosmos of science our knowledge must be nebulous before it constellates into definitely measurable shapes, and nothing is gained by attempting to anticipate the evolutionary process. Thus it is that here, for the most part, we have to content ourselves at present with the task of mapping out the field in broad and general outlines, bringing together the facts and considerations which indicate the direction in which more extended and precise results will in the future be probably found."