Whilst the greatest effort has been made to ensure the quality of this text, due to the historical nature of this content, in some rare cases there may be minor issues with legibility. I undertook this work some years since, but found that new observations were constantly requiring changes in and additions to what I had supposed was finished, till I despaired of complet ing the work to my own satisfaction. I have, however, taken the advice of a scientific friend, and arranged my observations for the press in their present form, conscious that much remains to be discovered relating to the objects of my study, and that many of my conclusions may require modification.