CHAPTER XVIII 

 THE VISION OF VERTEBRATES 



Of the many research workers who have given thought to the subject carl 

 VON HESS (1863-1923) (Fig. 735), Professor of Ophthalmology first at Marburg 

 in 1896, then at Wiirzburg in 1900, and finally at Munich in 1912, did more than 

 o-^.y other to put our knowledge of the visual behaviour of animals on a firm 

 basis. It is true that before the period of his active work much had been done 

 on this question, but no one up to his time had tackled this very difficult problem 

 with the same patience, zeal and enthusiasm. It is also trvie that some of his 

 conclusions are discredited today, particularly because of his habit of making 

 sweeping generalizations from experiments which time has shown to be some- 

 times uncritically founded ; but it is equally true that by the comprehensiveness 

 of his work, the ingenuity of the procedures he introduced and the diligence 

 with which he aj^plied them, he did more than any other to excite interest in 

 the visual life of animals and bring this subject within the ambit of science. In 

 this field his researches covered many aspects, particularly on the mechanism 

 of accommodation, the activities of the pupil, the light sense and, above all, the 

 colour sense, of a number of species. Xor did his interest end in comparative 

 physiology ; in pathology, both clinical and experimental, in bacteriology and 

 surgerj^ his contributions to ophthalmologj^ were immense. 



The Bole of Vision in Vertebrate Life 



It may be surprising to us who are markedly visual creatures and 

 whose most intimate contacts are with Mammals which apjDear to rely 

 largely on vision in their ordinary activities, that the great majority of 

 Vertebrates are much more nose- and ear-minded than eye-minded. 

 Yet such, indeed, is the case. Even the dog lives in a colourless world 

 of monotones in which, it is true, form-vision and luminosity count 

 highly, but its life is dominated to a very considerable extent by sounds 

 which we cannot hear and scents of the acuity and diversity of which we 

 have no conception. As we have seen to be the case with Invertebrates, ^ 

 for the mass of Vertebrates, not only phj^ogenetically and ontogenetic- 

 ally but also in daily life, the chemo-, the tacto- and the vibratory- 

 recejitors (the lateral line of Fishes and the ears of land animals which 

 have evolved therefrom) are more dominant than the eyes. Moreover, 

 it must always be remembered that even those species to which vision 

 is important, such as predators that hunt their prey, may possess visual 

 powers very different from our own; in many, reliance may be placed 

 almost entirely on the appreciation of luminosity and movement — not, 

 as in Birds and Man, on visual acuity — and this may serve them well. 

 In an attempt to reconstruct the visual world of animals it is easy to 

 fall into anthropomorphic mistakes of this type.^ 



1 Chaj^. XVII. - For a fuller discussion see p. IDS. 



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