466 ADAPTATIONS TO PHOTIC QUALITY 



titative basis of brightness. Nor may it be forgotten that two stimuli, 

 which diflFer for us in hue and in brightness, may be alike in both hue and 

 brightness for an animal, and still be distinguishable by him upon yet an- 

 other basis — saturation. It is on this basis, be it remembered, that the 

 dichromatic human distinguishes two 'yellows' which to the normal 

 appear respectively red and green. 



There is a certain amount of purely observational evidence for color 

 vision in some groups of vertebrates which, though it is no proof, is 

 strongly suggestive and did seem evidence enough to the elder school of 

 naturalists. Animals, particularly diurnal ones, are often brightly colored; 

 and there are often sexual differences which are either permanent or 

 nuptial — associated with the breeding season — as in some fishes, sala- 

 manders, lizards, and birds. Where the coloration is sexual, it is easy to 

 assume that it means color vision on the part of the opposite sex at least. 

 Indeed, this assumption was the very basis of Darwin's theory of sexual 

 selection. Where gaudy colors are not obviously involved in sexual recog- 

 nition one may assume that they indicate color vision on the part of the 

 animal's natural enemies — particularly where we can be at all sure of 

 concealing' or 'advertising' colors. 'Protective coloration' fools humans, 

 which it might not do if they were color-blind. It fools the animal's 

 enemies as well; and so, runs the argument, the latter must have color 

 vision, and moreover a system much like that of man. Color-changes to 

 fit backgrounds, while not nearly so common or precise as once thought, 

 could only mean the possession of color vision on the part of the changer. 

 Albino and isabelline birds are noticed, ostracized, and killed by their 

 normal relatives. Though this may imply only intensity-discrimination, 

 the hoarding habits of magpies, bower-birds, and others indicate a fas- 

 cination and ajsthetic interest in color as well as brightness. Bulls were 

 'well known' (before the days of experimental psychology) to be angered 

 by red objects. The mink is claimed by trappers to be very curious about 

 anything red, and it has seemed only natural that animals should be able 

 to distinguish at least this color, since spilled blood can be so important 

 to them. Finally, the coats of nocturnal birds and mamjnals tend to be 

 drab and dark. The tacit implication is that such colorations are prim- 

 itive, and that diurnal species have become dressed more gaudily because 

 somewhere there are eyes to be confused by, or to appreciate, the colors 

 which disappear — for all eyes alike — by the light of the moon. 



Observational evidence has been sufficient to fuel the fires of many an 

 argument between anglers, convinced or unconvinced that the color of a 



