THE EMERGENCE OF VISION 105 



ground too small to excite visual discrimination in training experiments. 

 Many of the reactions, as we have seen, are hormonal ; some may 

 occur in eyeless animals ; and indeed, in species wherein these organs 

 are necessary for their occurrence the chromatophores may still respond 

 if the eyes are transplanted to a new location in the body (as was 

 demonstrated in the adult fish, Fitzroya lineafa, by Szepsenwol, 1938). 

 Temperature and humidity, as seen in Amj)hibians and Reptiles, may 

 be equallj^ or more effective stimulants in comparison with light, and 

 although heat and light usually coincide in natural surroundings, the 

 paling of the desert lizard in the heat of noon so that it blends with the 

 sand is fortuitous so far as its own vision is concerned. Tactile organs 

 are sometimes adequate receptors as is seen in the control of chromato- 

 phores by the suckers of Cephalopods (Steinach, 1901) ; while the 

 adoption of a brown colour by the European tree-frog, Hyla arborea, 

 when it steps on a rough surface and of a green colour on a smooth 

 surface brings about an environmental adaptation to a background of 

 tree-bark or leaves respectively as adequate as any photic response. 

 Indeed, many of these colour reactions are fortuitous so far as adapta- 

 tion to a background is concerned ; thus the iguanid lizard, Anolis, 

 turns green in the shade and brown when exposed to light, and it is 

 merely coincidental that in its natural haiuits it usually becomes 

 invisible on a background of shady foliage in the first event or of soil 

 in the second, since, if it is removed from the shade upon a green leaf 

 and placed in the sun still sitting on the leaf, it promptly changes its 

 colour into a vividly contrasting brown (Wilson, 1939). 



It is essentially from the primitive motor response to light that 

 vision almost certainly developed. In natural circumstances these 

 tropisms and taxes are invariably of biological utility, and it would 

 appear that the essential and 'primary function of vision was the control 

 of movement iyi order to attain an optimum environment as efficiently as 

 jiossible, a function which is eventually employed for the avoidance 

 of obstacles, the pursuit of prey and flight from enemies, and survives 

 in man in the close relationship between the eyes and the vestibular 

 apparatus and in their importance in the control of posture. It follows 

 that visual organs are found almost solely in actively moving animals, 

 while in such as assume a sedentary phase they tend to degenerate 

 and disappear.^ 



The stage at which these motorial responses to light evolved 

 beyond purely reflex acts below the level of consciousness and became 

 endowecl with awareness is impossible to conjecture. This question 

 has given rise to a controversy which is still luisettled. 



In the simple philosophy of Aristotle - and for 2,000 years thereafter no 

 argument arose ; plants had a vegetative soul responsible for growth and repro- 



1 o. 721. - p. 28. 



