108 THE EYE IN EVOLUTION 



become merely relay-stations in this respect, they used to subserve 

 much more important functions. Indeed, in the higher animals — and 

 to some extent also in man — much of mental and most of visual 

 activity, especially those aspects associated with primitive responses 

 and endowed with emotional tone, remain closely associated with the 

 vegetative activities which are integrated in the thalamus. Even in 

 Fishes and Amphibians, vision is entirely unrepresented in the cortex. 



Thus although ablation of his occipital lobes deprives man permanently 

 and completely of all sensations of light, the higher mammals are by no means 

 so incapacitated. 1 Most decerebrate Vertebrates will react and exhibit emotions 

 to visual stimuli and even perform complex instinctive reactions without 

 difficulty. So will the headless bee sting with accuracy on irritation (Bethe, 

 1897) and the clover-fly clean its wings with its legs after decapitation 

 (Sherrington, 1920). A brain, or even a head -ganglion, is thus not a necessary 

 residence for apparently " intelligent " reactions. 



Phototactic reactions are " instincts", that is, adapted reactions 

 of a purposive nature handed down from the previous experience of 

 ancestors ; and, as with all instincts, the component afferent impulses 

 have become associated in consciousness and synthesized into a 

 meaningful pattern, a process which necessarily connotes some degree 

 of perception. 2 As instincts, their usual stereotyped uniformity can be 

 modified by experience provided the modification tends to the well- 

 being of the individual — or the race. The reactions of even the lowly 

 earthworm are amenable to training ^ ; many molluscs are readily 

 trainable ; many insects eminently so. Thus the photo -negative 

 cockroach, BlateUa gennanica, can be conditioned to advance towards 

 a light provided it has been taught that a dark and comfortable shelter 

 is placed beneath it (Goustard, 1948). Similarly, as we have seen,* 

 after interference with its receptors or effectors either by partial 

 blinding or by removing some of its legs, the mutilated insect will 

 rapidly modify its reactions and after several trials will learn to 

 orientate itself to light with almost the same accuracy as before. It is 

 thus impossible to say where in the animal scale reactions to light were 

 first associated with conscious awareness ; nor can we guess the form 

 such consciousness may take, for like a solid to an inhabitant of 

 Flatland, it exists in a form which cannot be assessed by the measuring 

 instruments at our disposal ; we can only reason by inference from an 

 analysis of our own peculiar form of consciousness of which alone we 

 have immediate knowledge. From a study of the sensory capacities of 

 animals few things emerge more certainly than that each species has 

 its own perceptual world (the MerhveU of v. Uexkiill, 1921), and that 



1 p. 545. 



2 See Lloyd Morgan (1896-1912), Jennings (1906), Sherrington (1920), Parsons 

 (1927), and manv others. 



» p. 573. " « p. 59. 



