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. Finally, questions which at the present time are still unsettled, and which occupy research, required to be stated, but it appeared to me to be the duty of a text-book to exercise reserve in relation to hypotheses which are as yet uncertain and to adhere strictly to the already assured ground of what is generally recognised.