628 



THE EYE IN EVOLUTION 



Bufo 



other hand, obtained entirely negative results in training experiments 

 involving coloured and grey papers with the toad, Bufo. 



Subjective training experiments being thus inconclusive, we are 

 left with evidence based on objective reflex responses. In this field 

 the work of Birukow (1937-50) who exploited the optomotor reaction, 

 is outstanding. Using this method he found that young tadpoles had 

 a maximum sensitivity in the yellow region of the spectrum at all 

 levels of illumination, while the adult frog, Rana, showed a Purkinje 

 shift with a maximum sensitivity in the yellow in light -adaptation and 

 in the green in dark-adaptation ; he also found that a specific colour 

 reaction could be obtained in this animal to red and blue, in the fire- 

 bellied toad, Bomhina, to yellow in addition, in the tree-frog, Hyla 

 arborea, only to blue, while the toads, Bufo and Alytes obstefricans, 

 showed no evidence of the possession of colour vision. Similarly, six 

 species of Urodeles {Salamandra and Triturus) exhibited evidence of 

 a differential response. Histological examination of all these Amphi- 

 bians showed a duplex rod-and-cone retina. From his experiments 

 Birukow concluded that in all cases the peripheral mechanism for colour 

 vision was equally present and that the lack in those species which 

 appeared to be colour-blind was in the central mechanism. When 

 colour-deficiency occurred it would seem that yellow and yellow-green 

 were the first colours to be missing, leaving a neutral region in the centre 

 of the spectrum, then red and blue -green, and finally blue. Whether 

 these reactions are associated with sensations is another question, and 

 in the meantime it would be wise to conclude that, although the 

 required mechanism may be present, there is little evidence that 

 sensations of colour enter prominently into the behaviour of 

 Amphibians. 



THE COLOUR VISION OF REPTILES 



Our knowledge of the colour vision of Reptiles is meagre, partly 

 because of the paucity of research done on the C][uestion and partly 

 because of the difficulty of using such animals as subjects in behavioural 

 experiments. The colour appreciation of Sphenodon has been 

 unexplored ; among the Crocodilians it would seem from the evidence 

 of pupillary contraction that a Purkinje shift occurs between a 

 maximum sensitivity of 544 m/z in light-adaptation to 514 niju, in dark- 

 adaptation (Laurens, 1923) ; while some snakes appear to be amenable 

 to colour training experiments (Kahmami, 1931 ; Grodzinska, 1948, on 

 the grass-snake, Troiyido7iotus). Sufficient work, however, has not been 

 done with these reptiles to allow us to draw any pragmatic conclusions. 



Some training experiments have yielded positive results with 

 Ohelonians and Lacertilians. v. Hess (1913) found that turtles showed 

 spontaneous colour-preferences, while Wojtusiak (1933), Quaranta 



