PREFACE 



vision, for example, is treated at particular length because no questions 

 are so often asked of the comparative ophthalmologist as those under 

 this aegis. Part II is an exposition of fundamental ideas rather than a 

 compendium of both explicable and at-present-useless facts. Because of 

 its ecological viewpoint, whole great fields find no place in it (or else- 

 where in this book) — ocular biochemistry, retinal photo-electrics, clinical 

 veterinary ophthalmology, most of physiological optics, and so on. 

 These chapters are intended to stimulate as well as to inform, and both 

 here and in Part III there is emphasis upon the more conspicuous of the 

 unsolved problems which await new students. 



Part III traces the history of the eye, group by group, from the lowest 

 living vertebrates to the highest. Here, place has been made for those 

 features which are of importance to the eye itself as a living thing, but 

 are not discemibly concerned in its performance in relation to the special 

 environment of its owner. The emphasis in these synoptic chapters is on 

 the morphology of the eye, the evolution of that morphology, and the 

 bearing of it upon the problems of vertebrate phylogeny. The animal as 

 a whole explains much about its eye, and in turn the eye can often 

 explain much about the animal. Thus, the structural plan of the snake 

 eye, its possible mode of origin, and the significance of this for the 

 evolutionary history of the snakes, are all interconnected matters. The 

 reader will find numerous sub-indices in Part III which will enable him 

 to round up quickly all the information about his favorite group which 

 has been given earlier in the book, and is omitted here to avoid dupli- 

 cation and waste of space. 



The illustrations have been kept as simple as possible, considering the 

 intricacies of the subject. Many are original, several of them — quite be- 

 yond my ability to make — beautifully drawn by the Misses Sylvia Hag- 

 yard and Gladys Larsen. Many others have been borrowed photograph- 

 ically from the journals, with or without changes (which are noted in 

 the legends) , and relabelled in accordance with a uniform scheme. Here, 

 much of the burden of work fell upon Albert Schlorff, without whose 

 expert photographic assistance I should have been quite helpless. I must 

 also acknowledge with gratitude the kindness of Viktor Franz in per- 

 mitting the free use of illustrations from his work. Figures 4, 5, and 41d 

 are by courtesy of William Bloom and the W. B. Saunders Company, 

 publishers of his 'Maximow's Text-Book of Histology'. Figures 6a and 

 16 are modified from Adler's 'Clinical Physiology of the Eye', by per- 

 mission of The MacMillan Company, publishers. 



