PREFACE 



a vertebrate eye, we should quite destroy its usefulness. Man can make 

 optical instruments only from such materials as brass and glass. Nature 

 has succeeded with only such things as leather and water and jelly; but 

 the resulting instrument is so delicately balanced that it will tolerate no 

 tampering. 



And yet, vertebrate eyes are not all alike — far from it. Each is a 

 cluster of harmonious parts, and the changes which have converted one 

 type of eye into another, through evolution, have necessarily involved 

 most of its parts. When one feature has had to be altered for some 

 primary ecological reason, this alteration has in turn called for con- 

 current secondary alterations of other structures, with the whole complex 

 remaining harmonious and workable at all times. Of course, many eyes 

 contain little odds and ends of structures which have no function. But 

 in every such case, one can be sure that the structure in question did not 

 arise in its present form, but is a vestige of a once important part which 

 is no longer needed, or whose task has come to be done better by some- 

 thing else in the eye. When such remnants are in the way — and they 

 usually are — the eye gets rid of them promptly, which may add greatly 

 to the difficulty of determining how the ocular pattern of a given group 

 was ever derived from that of a known ancestor. Fortunately, however, 

 there are few such gaps; and it is now possible to tell a well-connected 

 story of the evolution of almost any particular vertebrate eye. 



This book will be of particular benefit to zoologists and ecologists, 

 medical and veterinary ophthalmologists, and comparative psychologists. 

 But since none of these people speak the others' languages, I have been 

 able to assume no more scientific knowledge on the reader's part than the 

 contents of the usual elementary course in biology. The book should 

 therefore be entirely clear to any college student or graduate, and to any 

 amateur naturalist — 'trained' or not. As each unusual term has been 

 introduced, I have either defined it there and then or else placed it in the 

 glossary. The reader will find that the difficulty of the reading fluctuates, 

 which is inevitable in view of the varying weightiness of the material. 

 Some things about the eye and its workings are intricate, but I must 

 disclaim all responsibility for that — there are some subjects, such as 

 astrophysics and thermodynamics, which no writer could possibly 'pop- 

 ularize'. The reader will also soon note that my mode of expression is 

 strongly tainted with teleology. I do not expect this to mislead anyone — 

 it is merely an economy device, for it saves many words to say simply that 

 an animal has produced this feature or that to fill such-and-such a need. 



