PREFACE 



The material of the book is progressive, though this may not seem 

 to be indicated by the table of contents. I could not explain everything 

 at once, but I have so arranged matters that a given discussion will be 

 perfectly lucid if the reader has not skipped much before it. I hope, 

 naively of course, that anyone who reads in the book at all will read 

 the whole of it. It is not designed as a reference book, in which to 

 'look up' small points from time to time. Rather, it has been written 

 in the style of a text-book, though for a course which has yet to be 

 given in any American university. The book is not documented, i.e. 

 loaded up with specific citations for every point of fact and reasoning 

 which has originated outside of my own studies. The average reader 

 will not miss them; and the earnest student who reads the book, and 

 is led thereby to want to do research in its field, will have to devour all 

 of the required reading listed in the bibliography anyway. He — and 

 the established investigator in the field — will readily know which of my 

 pronouncements to blame upon me alone. If not, he is free to write to 

 me for specific bibliographic assistance, which I shall gladly furnish 

 within the limitations of my time and ability. 



Part I has been called 'basic' because it incorporates the first bodies 

 of information which the reader should have if he knows little or nothing 

 about the eye to begin with — even if he intends to skip straight to 

 Chapter 17 to find out what the pecten means. It is strongly urged that 

 every reader, even the ophthalmologist, read all of Part I before attempt- 

 ing to appreciate other chapters. In it, the human eye and human vision 

 have been used to acquaint the reader thoroughly with one sample eye 

 and its workings. The all-important retina is discussed in general terms. 

 The origins of the eye, ontogenetic and phylogenetic, are explained; and 

 the elementary facts of vertebrate inter-relationships are set forth so that 

 the non-zoological reader will understand the necessary taxonomic allu- 

 sions in Part II and the discussions of relationships and derivations 

 in Part III. 



Part II is the ecological body of the work. Here are gathered to- 

 gether, unoier the banners of various environmental factors, the evolu- 

 tionary responses of the vertebrate eye to those factors. In these chapters, 

 at some risk of cluttering, I have included many cross-references to 

 ensure that the reader who insists on dabbling will not miss information 

 pertinent to the satisfaction of his momentary curiosity. Some matters 

 are expounded in more detail than others, somewhat in proportion to 

 the interest I have found them to arouse — the subject of animal color 



