THE PRINCIPLES OF BIOLOGY BY HERBERT SPENCER _IN TWO VOLUMES_ VOL. II REVISED AND ENLARGED EDITION 1899 NEW YORK D. APPLETON AND COMPANY 1900 COPYRIGHT, 1867, 1899, BY D. APPLETON AND COMPANY. PREFACE TO THE REVISED AND ENLARGED EDITION OF VOL. II. To the statements made in the preface to the first volume of this revised edition, there must here be added a few having special reference to this second volume. One of them is that the revision has not been carried out in quite the same way, but in a way somewhat less complete. When reviewing the first volume a friendly critic, Prof. Lloyd Morgan, said:-- “But though the intellectual weight has also been augmented, it is an open question whether it would not have been wiser to leave intact a treatise, &c... relegating corrections and additions to notes and appendices.” I think that Prof. Morgan is right. Though at the close of the preface to volume I, I wrote:--“in all sections not marked as new, the essential ideas set forth are the same as they were in the original edition of 1864,” yet the reader who has not read this statement, or does not bear it in mind, will suppose that all or most of the enunciated conceptions are of recent date, whereas only a small part of them are. I have therefore decided to follow, in this second volume, a course somewhat like that suggested by Prof. Morgan--somewhat like, I say, because in sundry cases the amendments could not be satisfactorily made by appended notes. But there has been a further reason for this change of method. An invalid who is nearly eighty cannot with prudence enter upon work which will take long to complete. Hence I have thought it better to make the needful alterations and additions in ways requiring relatively moderate time and labour. The additions made to this volume are less numerous and less important than those made to the first volume. A new chapter ending Part V, on “The Integration of the Organic World,” serves to round off the general theory of Evolution in its application to living things. Beyond a new section (§ 289_a_) and the various foot-notes, serving chiefly the purpose of elucidation, there are notes of some significance appended to Chapters I, III, IV, and V, in Part IV, Chapters V and VIII, in Part V, and Chapters IX, X, and XII in Part VI. Moreover there are three further appendices, D^2, F, and G, which have, I think, considerable significance as serving to make clearer some of the views expressed in the body of the work.