RETINAL STRUCTURE AND ANIMAL HABITS 73 



Despite this point of view, this author still continues to dif- 

 ferentiate between rods and cones upon a morphological 

 basis. 



In relation to the duplicity theory Walls says, ''The occur- 

 rence of transmutation does not weaken the Duplicity 

 Theory as a statement of physiological facts, excepting the 

 Theory's allocation of hue perception exclusively to cones." 

 He says that the duplicity theory will have to be put to test 

 with respect to the implication that a cone which becomes a 

 rod loses hue-discriminatory capacity which it may have had 

 previously. Walls' (op. cit.) final statement is as follows: 

 ''No organon in all physiology is on a firmer foundation than 

 is the Duplicity Theory, in its true character as a description 

 of the functional antithesis of two physiological types of cells ; 

 but as a guide to the interpretation of habits in terms of 

 'structure,' the Duplicity Theory, faced with transmutation, 

 has come to the end of its usefulness." 



