218 SCIENCE PROGRESS 



piercing it, so to speak, with all manner of rents and vistas 

 of its brilliant sunlit background, utterly bewildering to the 

 beholder." In fact it becomes apparent that, if this principle 

 be transferred to butterflies, the inventors of similes between 

 those insects and what were supposed to be their human 

 prototypes have been sadly at fault. The aim of the butterfly 

 is not to flaunt itself before admiring spectators but to vanish 

 into its background and not to attract attention but to be 

 inconspicuous. The very caterpillar itself, with its speciality 

 of mimicry, is better material for the rhetoricians of social 

 denunciation than the maligned butterfly which does not 

 even, except in rare cases, pretend to be what it is not. 



Elsewhere Thayer works out in even greater detail the 

 concrete results of that averaging of the background which 

 really accounts for so much of the variety and brilliance of 

 animal colouration. Here is a particularly good specimen of 

 subtle analysis and delicacy of description devoted to the 

 American wood duck {Aix sponsa). " The general scheme 

 of this beautiful bird's disguisement includes a full potent and 

 obliterative shading from blue black on the back and tail to pure 

 white on the entire underside, shading through sand-colour 

 on the flanks and through chestnut mixed with white on the 

 breast. The throat is also white, ending abruptly against 

 deep velvet bronze and purple on the flanks. Founded on this 

 underlying obliterative shading, which conceals the bird's visible 

 solidity and prepares him for background matching, there is 

 a bright and beautiful system of water-pictures of many kinds, 

 bolder and more vivid than those of any other bird we know 

 (with the possible exception of Steller's eider). For the most 

 part these pictures are of shore and sky reflections, subtly 

 and richly intermingled and composing a great variety of 

 effects. The colours are mainly deep and soft, though rich and 

 liquidly alive with sober iridescence. Their range (excluding 

 the sandy flanks) is from chestnut red glossed over with purple 

 through all degrees of blue to golden green, perfect woodland 

 colours all of them. Most potent of all perhaps are the pictures 

 of reflections on the wood duck's richly crested green and purple 

 head, with its clean-cut shapes and bars of snowy white. 

 These white marks picture bright and sharp reflections of the 

 sky lying on the dark translucent water tinted by vague 

 reflections from the shore. Or again the white and dark marks 



