This book, originally published in 1871, has long been out of print, though in constant demand. Its publication was discontinued owing to the desire of the author to make certain emendations in the text, a work that the arduous activities of a professional scientific life left him no leisure to perform. A few changes, indicated by him, have been made. Otherwise the text of the present edition is that of the last, the revised and enlarged edition of 1874. Only the fastidiousness to which the extraordinary literary quality of the book is itself due, could suggest further modification of what is here republished with the motive of restoring to print and circulation a work too perfect in form and of too rare a quality to be allowed to lapse. It is accordingly with the view of renewing the accessibility of a genuine classic of American literature that the present edition is presented.