Turning from the additions to the revisions, I have to say that the aid needed for bringing up to date the contents of this volume, has been given me by the gentlemen who gave me like aid in revising the first volume: omitting Prof. Perkin, within whose province none of the contents of this volume fall. Plant-Morphology and Plant-Physiology have been overseen by Mr. A. G. Tansley. Criticisms upon parts dealing with Animal Morphology I owe to Mr. J. T. Cunningham and Prof. E. W. MacBride. And the statements included under Animal Physiology have been checked by Mr. W. B. Hardy. For reasons like those named in the preface to the first volume, I have not submitted the proofs of this revised second volume to these gentlemen: a fact which it is needful to name, since one or other of them might else be held responsible for some error which is not his but mine. It is the more requisite to say this because while, in respect of matters of fact, I have, save in one or two cases, accepted their corrections as not to be questioned, I have not always done this in respect of matters of inference, but in sundry places have adhered to my own interpretations. Perhaps I may be excused for expressing some satisfaction that I have not been obliged to relinquish the views set forth in 1864–7. The hypothesis of physiological units--or, as I would now call them, constitutional units--has been adopted by several zoologists under modified forms. So far as I am aware, the alleged general law of organic symmetry has not called forth any manifestations of dissent. The suggested theory of vertebrate structure appears to have become current; and from the investigations of the late Prof. Cope, has received verification. The conclusions drawn in Part VI on “The Laws of Multiplication,” have not, I believe, been controverted. And though only some works on botany have given currency to the doctrine set forth in Appendix C, “On Circulation and the Formation of Wood in Plants,” yet I have met with no attempt to disprove it. The only views contested by certain of the gentlemen above named, are those concerning the origin of the two great phænogamic types of plants, and the origin of the annulose type of animals. I have not, however,--perhaps because of natural bias--found myself compelled to surrender these views. My reasons for adhering to them will be found in notes to the ends of Chapters III and IV in Part IV, and in Appendix D^2. On now finally leaving biological studies, it remains only to say that I am glad I have survived long enough to give this work its finished form. BRIGHTON, _October, 1899_. PREFACE TO VOL. II.