DUPLICITY AND TRANSMUTATION 163 



primarily, strange to say, for the regulation of intra-ocular illumination. 

 The unique physiological role of the seal's pupil will be found explained 

 on pages 446-8. 



In the mammals, the retinal photomechanical changes are entirely 

 gone. In this group of vertebrates we see the end result of the evolution- 

 ary replacement of those older equalizing devices by the more rapid, 

 hence highly superior, one afforded by the iris musculature. 



(D) Duplicity and Transmutation 



The duplex retina itself is clearly an adaptation for the extension of 

 the seeing-period over a greater number of the twenty-four hours. Rods 

 and cones are homologous inter se, and one type must have preceded 

 the other in evolution; for, an intermediate type of visual cell partaking 

 equally of the qualities of modern rods and cones is quite impossible 

 of conception. 



The accepted belief is that the rod is the more ancient and that the 

 cone is an improvement upon it; but what real evidence there is points 

 to the exact reverse of this view. The problem involved here reminds one 

 of the question: "Which came first, the hen or the egg?" but it is not 

 without theoretical importance in connection, particularly, with color 

 vision. The evidence derives largely from the embryology of the visual 

 cells as interpreted in phylogenetic terms. There is not space here to set 

 it forth in detail, so the designation of the cone as primitive is bound to 

 seem a little arbitrary. 



Considering the flagellar origin of the outer segment (see Fig. 55, 

 p. 127) the percipient parts of ancient visual cells must have been 

 filamentous before they could have become massive, and we cannot 

 imagine the pro-vertebrate to have possessed already so ingenious a 

 material as rhodopsin or to have been anything but a strictly bright- 

 light, pelagic organism. Until some of the visual cells became enlarged, 

 and grouped in their connections to ganglion cells, there could be no 

 increase in potential sensitivity which could release the animal from 

 bondage to the sun; and until the invention of rhodopsin, there could 

 be no visual activity by moonlight. But withal there must be no whole- 

 sale conversion of visual cells if the capacity for daytime activity was 

 to be retained — else the animal would merely have succeeded in shifting 

 the active period without substantially extending it. Improvements in 

 the dioptric apparatus making it more and more desirable, for the sake 



