THEORIES OF PHOTOMECHANICAL CHANGES 91 



Walls (1928) suggested that correlated with the backward 

 orthogenesis in the pigment migration is a forward progress in 

 the development of the iris reaction. He calls attention to the 

 fact that in fishes " ... the pupil is practically stationary; 

 in the amphibians it expands and contracts within narrow 

 limits ; in reptiles the response is about the same as in amphib- 

 ians, while in mammals and man the response is very rapid 

 and very extensive." Walls thus assumes that the pupillary 

 reaction is a much more efficient mechanism for protecting 

 the retina against too much light than are the sluggish pig- 

 ment and cell movements. As valuable as this suggestion 

 would seem, it may be pointed out that in birds, where the 

 iris response is more rapid than it is in the mammals, the 

 pigment is not only abundant, but undergoes extensive 

 migration. Furthermore in the gecko which is entirely noc- 

 turnal, and which possesses a vertical slit pupil, rapidity of 

 the iris reaction surpasses that of the human and approaches 

 that of birds (Rochon-Duvigneaud, 1917), yet here too we 

 find very heavy pigment processes surrounding the outer 

 segments of the rods (Figure 41). Walls (op. cit.) recognizes 

 this condition in the birds, and says: ''It must be admitted 

 that the situation in nocturnal animals where both kinds of. 

 phenomena are lacking, and in birds, where both kinds of 

 phenomena are present to a marked degree, is disturbing 

 to this suggestion, but the nocturnal animals have little or 

 no need for either mechanism, and the eyes of birds are so 

 aberrant in many respects that perhaps we are safe in dis- 

 missing them in this instance." It hardly seems justifiable 

 to the author to regard the birds' eyes as being so aberrant. 

 They possess a double retina, many have a fovea, some have 

 a double fovea, and they are known for their high visual 

 acuity. With respect to their structural characteristics they 

 are more typical than are the eyes of many other animals 

 where, for example, only one type of visual cell is present. 

 Furthermore, facts do not bear out the statement that 

 nocturnal animals have little or no need for a pupillary 

 mechanism. 



