I must explain at once that these few pages do not attempt or pretend to be anything like a formal review of the recently published posthumous volume of Professor Bowman with the same title. I am precluded from writing such a review partly by the wide range of problems attacked by the author, partly by my own insufficient familiarity with many of the positions of the most recent physical and natural science which are brought under review. I will therefore confine myself, so far as the strict business of the reviewer is concerned, to the single remark that the editor, Professor J. W. Scott, has discharged his difficult task of preparing the book for publication—no easy matter, as will be seen from his Preface—with equal skill and devotion, and has laid himself open to no worse criticism than that there are less than a dozen obvious slight misprints which have escaped detection, but will readily be detected by a careful reader. What I propose to do in the remarks which follow is simply to indicate the very real importance of the book by saying something as to its main purpose and thesis, and the points where I still feel that there is some difficulty or ambiguity about the writer's position which would, no doubt, have been largely cleared up if he had lived to reconstruct the whole six of his Vanuxem Lectures for publication as he has done the first three.