Transcriber’s notes:

The text of this complex e-book has mostly been preserved in its original form, including inconsistent punctuation, spacing, capitalisation, hyphenation and diacritics; however, some such flaws have been corrected silently. A list of spelling corrections is appended at the end of the book. Multiple levels of headings and subheadings, not all of which are consistently formatted, can make it difficult to follow the text coherently. The author has used [square brackets] to identify his personal additions but these are not always readily detectable and can be confused with similar brackets used for other purposes.

The book contains numerous illustrations that vary in size and complexity, and many have been repositioned to place them nearer the relevant text. The List of Illustrations therefore gives only an approximate indication of their location. Depending on the size of the viewing screen the text flows around them to a variable extent, particularly noticeable with small-screen e-readers where illustrations and their lengthy captions may be partly or completely separated.

Footnotes have been renumbered consecutively and moved to the end.

One incorrect index page reference has been changed, viz. 730 → 369 ‘pigment of the skin’

New original cover art included with this e-book is granted to the public domain.