256 THE EYE IN EVOLUTION 



the Primates. In these three groups alone is a fovea found making 

 possible a good acuity of vision ; in these, highly developed accommo- 

 dative mechanisms are present allowing accuracy of form vision over a 

 wide range of distances ; and in these alone good colour vision has 

 been demonstrated. In each of these the optic axes may be swung 

 forwards so that the visual fields are made to overlap, thus rendering 

 it possible for binocular to replace panoramic vision ; in the last group 

 a partial decussation of the optic nerve fibres allows an anatomical 

 basis for the coordination of ocular movements ; and finally, a neo- 

 pallium built up upon the sense of vision replaces the original archi- 

 pallium which was based upon the sense of smell. In this way the 

 dyscritic mechanism of the simple eye of the lower Vertebrates, which 

 was essentially adapted to the biologically primitive function of the 

 appreciation of light and movement, developed the capacity for the 

 intelligent appreciation of complex visual patterns and the potentiality 

 to form reasoned visual judgements. 



The interesting thing is that the eye of each of these types has 

 developed separately and independently ; between them there is no 

 evolutionary sequence, for all have attained their high degree of 

 efficiency by different expedients which, when they show affinities, owe 

 their relationship to the fact that they have evolved not the one from 

 the other, but all from the same original substrate of physiological 

 potentialities. It is also interesting that of these types the sauropsidan 

 eye is the most efficient as an optical mechanism ; of all the three, 

 Birds have relatively the largest and absolutely the most specialized 

 eyes, tlie most efficient focusing apparatus, a pecten structure instead 

 of a retinal system of vessels, the most complex macular arrangements, 

 and the highest visual acuity. The eye of man cannot therefore be 

 considered as representing the acme of efficiency as an optical instru- 

 ment ; it is to the unique and transcendent development of the 

 associated cerebral centres that it owes its functional predominance. 



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