CHAPTER V 

 THE EMERGENCE OF VISION 



In the varying reactions of living organisms to light that we have 

 now studied, in some cases vision does not — or need not — co-exist, in 

 others an associated sensory impression is conjectural and unimportant 

 while in others it seems to be a necessary accompaniment ; indeed, it 

 is no easy matter to decide where its origin lay or when the sense of 

 vision first became a factor in conscious behaviour. There are many 

 creatures which have no eyes (as we understand the term) and yet 

 " see " (using the word in its widest sense) ; and equally reasonably it 

 may be said that there are many which have what we may well call 

 " eyes " and yet see not. 



To a considerable extent the matter is one of definition ; on the 

 one hand, few would acquiesce with Max Schultze (1868) who spoke of 

 the transformation of luminous into nervous energy as vision ; more 

 would agree with Hesse (1908) who contended that the light-sensitiveness 

 of primitive creatures did not imply the possession of a light sense. On 

 the other hand, there are those w^ho would ascribe to all animals which 

 react to light a sentiency, no matter how vague (McDougall, 1933). 

 To many this may seem gratuitously anthropocentric ; for if such an 

 awareness, tinged with affective tone, is ascribed to the amoeba as it 

 flees from a bright light and expands in mid-intensities of illumination, 

 is it to be ascribed also to the speedwell which opens its petals to the 

 mid-morning sun? The question is disputable ; but whichever attitude 

 we adopt the most illegitimate premise from which we can reason is the 

 assumption that an organism has the same appreciation of light and 

 patterns of shade or hue as ourselves, whether it reacts diffusely without 

 specific end-organs or whether it is possessed of eyes more highly 

 differentiated for the resolution of visual images than the relatively 

 simple eyes of man.^ 



It must be remembered, however, that vision is one of the latest 

 senses to be evolved and that in its phylogenetic development it 

 lingered long behind those depending on mechano -receptors and 

 chemo-receptors. Even when a considerable stage of complexity had 

 been reached there was little attempt at discrimination ; for this 

 purpose reliance was placed upon those senses which are more fully 

 developed in primitive life — the tactile sense, the chemical sense, and 

 the olfactory sense. The great majority of animals are non-visual 



' The few sicjn-stimuli to which the vision even of birds is limited are striking 

 examples (p. 664 , 



