618 



THE EYE IN EVOLUTION 



Polecat 



positions were changed after each trial (Perkins and Wheeler, 1930 ; 

 Perkins, 1931). Within limits, therefore, an appreciation of differences 

 of intensity is possible to Fishes. 



Similarly, in training experiments Wojtusiak (1933) found that 

 turtles had great difficulty in distinguishing shades of grey. According 

 to the findings of Hamilton and Coleman (1933) training experiments 

 showed that a diurnal bird (the pigeon) is more attentive to changes 

 in hue than in brightness ; while in most Mammals the opposite 

 obtains. Among these, in most of the nocturnal types which have 

 been investigated, the discrimination of brightness has been found to 

 be excellent (Cole and Long, 1909, in the raccoon ; D. Miiller, 1930, in 

 the polecat ; Munn, 1932, in the rat) ; in the guinea-pig, however, 

 Sgonina (1936) found that the intensity of two greys had to differ by 

 1/3 before differentiation of them could be made. Among diurnal 

 types Salzle (1936) found that the discrimination of brightness was 

 poor. In arhythmic types, on the other hand, it may be very good 

 indeed. Thus Orbeli (1909), eliciting conditioned reflexes in salivary 

 secretion in the dog, found that this animal was capable of differentiat- 

 ing perfectly between closely related shades of grey {e.g., between 49 

 and 50 of the Zimmermann scale) which are quite indistinguishable 

 to the human eye, whether they were presented successively or 

 simultaneously. Indeed, so far as the dog is concerned, Pavlov 

 (1911-27) concluded that the analysis of the intensity of illumination 

 is so highly developed that a human experimenter is unable to determine 

 its limits. 



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