The general purpose I have had in view in writing the present treatise will be gathered from the introductory pages; but I wish to offer here a few remarks on certain points of detail.

It will be seen that on many of the subjects dealt with in this work, I have propounded views which differ from those usually accepted. I have not done this from any love of novelty, nor from any desire to attract attention by bizarre or fanciful theories. Each of the new views here presented has been the result of a careful study of the subject dealt with, and I have searched as anxiously for considerations opposed to any novel theory, as for arguments in its favour. If others should be more successful than I have been in finding reasons for rejecting any of my views, I shall be ready to abandon them without regret. I trust I am free from that weakness which forces a man to regard every theory he has once advocated as a matter to be defended at all hazards. No weakness more mischievously affects the work of the student of science.