INTRODUCTION 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. Ihave 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 appéal 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 
