374 STUDIES IN SPECIAL SENSE PHYSIOLOGY 



essentially similar to the one we are considering. Kiihne subse- 

 quently showed ( 27 ) that visual purple was present only in retinae 

 containing rods. He was not, however, able to extract this pig- 

 ment from all rod-containing eyes, a notable exception being the 

 bat ; recently, Trendelenburg ( 28 ) has obtained abundance of visual 

 purple from more than one species of bat. Experimentally, as we 

 have seen, AbelsdorfT has shown that the owl is specially sensitive 

 to short waves and the dove relatively insensitive (tested by the 

 pupillo-motor response). 



We all know that most nocturnal animals see badly in broad 

 daylight, while such birds as the pigeon exhibit normally a marked 

 degree of night-blindness. In Parinaud's words : 



"It is a matter of common observation that hens and pigeons 

 see very imperfectly in artificial light and defend themselves with 

 difficulty against the hand that tries to seize them ; that as soon 

 as the sun goes down these animals seek their night shelter. The 

 old adage, ' To go to bed with the hens,' meaning to go to bed 

 early, evidently having its origin in this fact " ( 10 , p. 66). 



Biological investigation appears to show, therefore, a co- 

 existence of rods and visual purple with vision of the twilight 

 variety and of cones with optimal vision in daylight. It is easy 

 to lay too much stress on this sort of evidence. Reasoning from 

 analogy is dangerous and especially equivocal when dealing with 

 sense physiology. Take the history of opinion regarding a bat's 

 vision. The objectors to the theory of duplicity pointed out that 

 no visual purple had been extracted from the bat's retina. Its 

 partisans retorted that the bat relied on sensory mechanisms other 

 than sight, possibly scent currents, as seems to be the case with 

 moths. My friend Mr. Arthur Bacot points out to me that the 

 . remarkably rapid darting movements of a bat in pursuit of prey, 

 movements which are comparable in point of velocity with the 

 swift's flight, seem hardly compatible with the type of twilight 

 vision we have agreed to associate with the rods, characterised as 

 that is by poor acuity of vision. We are now aware that many 

 species of bat possess visual purple. Hence, if the previous train 

 of reasoning be at all correct, the value of an outfit of rods and 

 visual purple to the twilight animals is rendered doubtful. In 

 precisely the same way, the early roosting of diurnal birds may be 

 due to causes other than a condition of night-blindness. 



It is necessary to dwell upon these points because in no de- 



