630 



THE EYE IN EVOLUTION 



Sparrow 



The somewhat surprising suggestion was, however, put forward by 

 Vanderplank (1934) that the pupil of the tawny owl, Strix, contracted to long 

 infra-red rays (900 mjA) far beyond the limits of human visibility, the idea being 

 that this bird " saw " its prey in the dark by means of the latter's body-heat. 

 This, however, has been refuted by Hecht and Pirenne (1940) in another species 

 of owl, Asio, while Matthews and Matthews (1939) showed that the ocular media 

 of Strix absorbed completely all the infra-red radiation in this spectral area. It 

 may therefore be accepted that the objective evidence indicates that the photo- 

 chemical system of the eyes of Birds is similar to that of man. 



Behavioural experiments have borne out the same conclusion in 

 a very definite way although they have been somewhat handicapped 

 by the essential stupidity of birds. That colour vision does influence 

 their behaviour was shown by the early experiments of Lloyd Morgan 

 (1896) with chickens, Porter (1904-6) with the sparrow and Rouse 

 (1906) with the pigeon ; these birds all show a preference for certain 

 colours and can be trained by food-association to pick them out. The 

 most elaborate investigations, however, were undertaken by Carl von 

 Hess (1912) who experimented both with diurnal (chickens, pigeons) 

 and nocturnal birds (owls). He found that chickens, for example, 

 picked up grains of rice illuminated on a white ground by spectral 

 lights from the red end of the spectrum to the green but refused those 

 illuminated with blue light ; he therefore concluded that this bird had 

 colour vision but that the spectrum was much shortened at the short - 

 waved end and that the fowl was blue-blind. It is interesting in this 

 connection that in contrast to the yellow, blue or white flowers pre- 

 ferentially pollinated by bees,i the usual bird-pollinated flowers are red 

 (Werth, 1915 ; Pickens, 1930 ; Porsch, 1931). This suggestion of 

 blue -blindness, although supported by Heiming (1920), has not stood 

 the test of time, for it has been subsequently shown that the hen and 

 many other species can see blue and violet, but that training is necessary 

 if the bird is not to reject a food coloured quite unlike anything in 

 nature ; there is, however, a certain degree of blue -violet -weakness, 

 probably because of the absorption of short-waved light by the retinal 

 oil-droplets (Watson, 1915 ; Lashley, 1916 ; Halm, 1916 ; Honigmann, 

 1921 ; Blasser, 1926 ; Bailey and Riley, 1931 ; Hamilton and 

 Coleman, 1933 ; Plath, 1935 ; and others). It would seem clear that 

 the limits of spectral visibility and the discrimination of hues resemble 

 those of man ; that the colour-vision system might possibly be 

 interpreted on a trichromatic basis^; that a relatively small number of 

 hues are distinguishable (20 by the pigeon in contrast with 160 by 

 man, Hamilton and Coleman, 1933) ; while by training birds to peck 



1 p. 587. 



* It is always to be remembered that by trichromatic vision is meant the ability to 

 match all colours with a mixture of three, and only three, primary colours. This must 

 involve colour-mixing experiments and without these it is illegitimate to draw any 

 conclusions as to the number of mechanisms involved. 



