THE EMERGENCE OF VISION 109 



each of this midtitiide of worlds bears Httle resemblance to the environ- 

 ment of the animal as we see it or interpret it in terms of our own 

 Merkwelt. 



It seems reasonable to assume that the development of vision as a 

 facet of consciousness evolved in three stages. We may surmise that 

 the first conscious appreciation was a mere sentiency, crudely vague 

 and undifferentiated, characterized perhaps by a minimum of cognition 

 endowed with a rudimentary affective tone ; it was limited perhaps to 

 an awareness of the existence of light as a change in the environment, 

 tinged perhaps with sufficient affective tone to allow it to be appreciated 

 as pleasant or mipleasant, and endowed witli meaning in so far as the 

 organism responded apjDropriately by motor activity in which initially 

 there was offered the choice only of two alternatives, towards or away 

 from the source of stimulation. We may even surmise^ as indeed exjjeri- 

 mental evidence on the amoeba would suggest,^ that the most primitive 

 sensation was a co-sesthesis without constituent modalities in which the 

 several senses as we know them were merged into a vague and indis- 

 criminate unity, and the stimuli (photic, chemical, tactile, etc.) which 

 to us are distinct and unrelated were co-equal and additive. Some such 

 concept as the emergence of a consciousness of a lowly type at an early 

 but unknown stage, on the reflex plane or even below, would seem 

 a possible hypothesis, a consciousness at first indefinable and vague 

 but at the same time sufficiently plastic to contain the germ of the 

 elaborate emotional behaviour of the higher animals — so long as we 

 remember that the latter with all its undoubted richness and com- 

 plexity bears little resemblance to the consciousness of man. 



For such a surmise, however, there is no direct evidence; at this 

 level the motor response to stimulation is all we can directly assess. 

 From morphological and behavioural observations, however, we can 

 be more certain that a primitive perception of light emerged with the 

 development of a centralized nervous system in worms - ; at this stage 

 in evolution it would seem reasonable to suppose that a mechanism 

 became available for the creation of perceptual symbolism; and at 

 this stage vision undoubtedly became a perceptual process forming part 

 of the conscious life of the animal and capable, at first in a minor 

 degree, of determining its conduct. As we ascend the animal scale the 

 primitive light-sense evolved into a sense of appreciation of the 

 directional incidence of light, of movement, of form, and eventually 

 of colour, until in the Primates the capacity to analyse complex 

 visual patterns ])ecame the chief determinant of conduct. In its final 

 development, the first elements of which have been detected in the 

 chimpanzee,'* the sense of vision j^assed beyond the stage of passively 



1 p. 3G. Compari' also the integration of jjliototaxi.s and galvaiiotroi^ism seen in 

 certain worms (p. 33). ^ p. 572. ^ p. 602. 



