The highly speculative and extrapolatory character of this book will be evident to all who are bold enough to read it. I wish to make it perfectly clear that I have no intention of dogmatising on so Obscure a subject. The suggestions which follow are purely tentative, and I am well aware that some of them are likely to prove mutually incompatible. But it is only by the bold formulation and ruthless rejection of hypotheses that progress is made, and even if we are compelled to abandon the Higher Space Hypothesis altogether — as is very possible the negative information so gained will be of the greater value if the hypothesis has first been given the fullest possible trial.