632 THE EYE IN EVOLUTION 



eliminates most of the violet and some of the blue, thus diminishing glare and 

 at the same time considerably reducing chromatic aberration, a phenomenon 

 for which these wave-lengths are partly responsible. This colour of droplet is 

 thus found preferentially in the central and ventral parts of the retina where it 

 mvist increase acuity and enhance contrast by eliminating the preponderating 

 and dazzling blue light of the sky. The reduced sensitivity to blue light of the 

 chicken and other species remarked vipon above is thus explained, as well 

 (perhaps) as the tendency of such birds as pollinate flowers (humming-birds, 

 honey-birds) to choose preferentially red blooms. The red droplets, cutting off 

 the spectrum at 580-590 mfj,, will be particularly valuable in damping down the 

 excessive long-waved light at sunrise or sunset (hence their large numbers in 

 early-rising song birds in which they comprise 20% of the total) ; they will have 

 a similar effect on the light of long wave-lengths reflected from the water (hence 

 their presence in quantity in turtles or the kingfisher) ; they will be of less value 

 in other optical conditions (hence their paucity — 3 to 5% — in late-rising cre- 

 puscular types such as swifts or swallows) ; while their preferential occvirrence 

 in the dorsal part of the retina will give maximal contrasts to objects seen against 

 a green backgrovind. The orange droplets will provide a transition between the 

 two. 



It would seem, therefore, that the oil-droplets have no part in the 

 mechanism of colour vision, but at the same time they must influence 

 the appearance of coloured objects so that the bird's appreciation of 

 them ought to be quite different from ours. To birds that possess them, 

 central vision probably resembles vision through yellow spectacles, 

 while elsewhere in the retina with the constant sudden movements of 

 the head, each object is scanned and analysed now through one filter, 

 now through another, the kaleidoscopic changes allowing an unusually 

 high discrimination of tone and necessarily increasing contrast and 

 therefore the visibility of details.^ Looking through this polychromatic 

 mosaic, a bird should be able to distinguish objects invisible to us : 

 thus Judd found that a bird could readily pick up crickets mixed 

 deliberately with dry leaves although he could not differentiate between 

 them, while Rabaud (1920) noted that sparrows saw at once and ate 

 green phasmids, which as far as he was concerned mimicked perfectly 

 the leaves on which they rested. 



THE COLOUR VISION OF MAMMALS 



As we would expect from the rarity of strongly diurnal Mammals, 

 the possession of colour vision by the members of this class is apparently 

 rare. The more primitive and nocturnal types are colour-blind; some of 

 the arhythmic types may possess some degree of hue-discrimination but 

 this faculty plays a small part in their behaviour, being completely 

 subservient to sensations of luminosity ; only the higher Primates 

 have a colour sense sufficiently developed to influence their activities 

 to an extent that can be experimentally elicited with certainty. 



Of the colour vision of monotremes we know nothing ; among 

 MARSUPIALS, Salzle (1936) reported negative results with the opossum, 



^ See further p. 662. 



