{5}It is now twenty years since the theory first came into my mind, and the work of those twenty years has convinced me more and more of its truth, and yet during the whole time it has been ignored by the morphological world as a whole rather than criticized. Whatever may have been the causes for such absence of criticism, it is clear that the serial character of its publication is a hindrance to criticism of the theory as a whole, and I hope, therefore, that the publication of the whole of the twenty years' work in book-form will induce those who differ from my conclusions to come forward and show me where I am wrong, and why my theory is untenable. Any one who has been thinking over any one problem for so long a time becomes obsessed with the infallibility of his own views, and is not capable of criticizing his own work as thoroughly as others would do. I have been told that it is impossible for one man to consider so vast a subject with that thoroughness which is necessary, before any theory can be accepted as the true solution of the problem. I acknowledge the vastness of the task, and feel keenly enough my own shortcomings. For all that, I do feel that it can only be of advantage to scientific progress and a help to the solution of this great problem, to bring together in one book all the facts which I have been able to collect, which appeal to me as having an important bearing on this solution. In this work I have been helped throughout by Miss R. Alcock. It is not too much to say that without the assistance she has given me, many an important link in the chain of evidence would have been missing. With extraordinary patience she has followed, section by section, the smallest nerves to their destination, and has largely helped to free the transformation process in the lamprey from the mystery which has hitherto enveloped it. She has drawn for me very many of the illustrations scattered through the pages in this book, and I feel that her aid has been so valuable and so continuous, lasting as it does over the whole period of the work, that her name ought fittingly to be associated with mine, if perchance the theory of the Origin of Vertebrates, advocated in the pages of this book, gains acceptance. I am also indebted to Mr. J. Stanley Gardiner and to Dr. A. Sheridan Lea for valuable assistance in preparing this book for the press. I desire to express my grateful thanks to the former for valuable criticism of the scientific evidence which I have brought {6}forward in this book, and to the latter for his great kindness in undertaking the laborious task of collecting the proofs. LIST OF PREVIOUS PUBLICATIONS BY THE AUTHOR, CONCERNING THE ORIGIN OF VERTEBRATES. 1888. "Spinal and Cranial Nerves." _Proceedings of the Anatomical Society_, June, 1888. _Journal of Anatomy and Physiology_, vol. xxiii.