104 THE EYE IN EVOLUTION 



It must also be remembered that even although vision is well developed it 

 may not be used in many innate reactions for the efficient execution of which 

 it would appear to us to be of value. The feeding response of the carnivorous 

 water-beetle, Dytiscus marginalis, is a good example of this (Tinbergen, 1936) 

 (Fig. 74). This beetle has elaborately developed compound eyes and can be 

 trained to respond to visual stimuli. Its feeding response, however, is released 

 only by chemical and tactile stimuli, and visual impressions, even those of a 

 moving prey, never release this reaction. Thus in the presence of a watery meat 

 extract it neglects the source but, going to the region of highest concentration, 

 it attacks any solid object it touches. 



Of the three fundamental effects of hght on Hving organisms — the 

 stimulation (or occasionally the depression) of metabolic activity, the 

 orientation of movement, and the control of pigment and colour — it 

 would he reasonable to assume that the first, equally shared between 

 plants and animals, does not necessarily involve vision as a conscious 

 experience, occurring as it does in Protozoa and eyeless types. ^ In its 

 more primitive form this activity may conjecturally be accompanied 

 by a vague sentiency, but this can be little more than an awareness of 

 light, and even in its most advanced forms it is essentially a chemical 

 or hormonal function for the implementation of which eyes are effective 

 but not unique receptors. The last — the control of colour — is a late 

 evolutionary development, and although j^oikilochromic reactions 

 would appear to occur without conscious accompaniment, in their 

 higher developments they would seem to imply the existence of a visual 

 sense in the organism for whose benefit (or confusion) they are intended. 

 The economy would seem unnatural and contrary to all biological 

 trends that at one time urged all plants except the modest Cryptogams, 

 in their struggle for existence in a cooling world, to luxuriate so shame- 

 lessly in the blatant sexual exhibitionism of flowering if the pollinating 

 insects could not both see and appreciate their charms ; their appre- 

 ciation, however, has probably no resemblance to the interpretation 

 of the same imagery by the human brain. Equally uneconomic would 

 be the scandalously attractive dress put on by many fishes and birds 

 for tlieir love-making. Clearly, if they are endowed with biological 

 usefulness and survival-values, allsesthetic characters — and without 

 these endowments they would not jiersist — must be appreciated by 

 other organisms. 



Although the eyes serve as the receptors for many adaptive colour 

 changes, this function need not imj^ly that the animal it'self has any 

 conscious appreciation excited by shifting visual patterns. Even when 

 the responses are mediated nervously and are rapid and complex, as in 

 teleostean fishes, they show no parallelism with what is known of the 

 visual functions of the animals concerned, for reflex alterations of the 

 chromatophores may occur to suit differences in shade of the back- 



' Such, for example, as the white cave crayfish, Cambarus ayersii (Wells, 1952). 



