SIGNIFICANCE AND DISTRIBUTION 149 



If the animal now enters darkness or even a dimly-lighted situation, 

 the movements proceed, more slowly than in light-adaptation, to reverse 

 themselves : the pigment granules glide back up out of the processes and 

 concentrate as a dense band in the cuboidal cell-bodies of the epithelium, 

 the rod myoids shorten and draw the sensitive outer segments away from 

 the pigment and thus toward the light, and the cone myoids elongate to 

 push the cone bodies toward the pigment — sometimes to no apparent 

 purpose, but in some animals thereby making appreciably more room for 

 the rods to gather in a smooth layer close to the limitans (Figs. 62, 63, 

 and 64). 



Significance and Distribution — Where the photomechanical changes 

 are as complete as described above, and carried out smoothly and within 

 an hour's time or less, the whole machinery is clearly of great value in 

 adjusting the retina to the external illumination. The workings of the 

 Duplicity Theory are beautifully seen in these phenomena, for the cones 

 are most advantageously placed for action in bright light, the rods being 

 then shielded from excessive stimulation (or from any at all) ; and in 

 dim light the rods are fully exposed while the cones get out of their way, 

 whether this latter happening has any obvious value or not. As a device 

 for equalizing the actual stimulation permitted to the visual cells in 

 various illuminations, the photomechanical system at its best is excellent 

 and has only the single drawback of slowness. Even this defect may be 

 unimportant in the case of an animal with sedentary habits and deliber- 

 ate movements, for temporal changes in natural illuminations are rarely 

 rapid. But the low speed of the retinal migrations would seem to be 

 detrimental to an agile species which flits from light to shade sporadic- 

 ally and lacks any more rapid means of regulating the illumination of 

 its visual cells. Among such forms would be fishes which move rapidly 

 from the bright surface to the dim depths and vice versa, and those 

 which inhabit coral reefs and the like, which may help to explain why 

 the latter are commonly crepuscular. 



Table II summarizes the occurrence and relative effectiveness of the 

 photomechanical changes in the various vertebrate groups. The reader 

 will note a general tendency for them to dwindle in importance as one 

 passes from lower to higher forms, the reason for which will be discussed 

 in Section C. 



