The Wit of the Wild 



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old lichen-covered fence-rail, a moldering log 

 or variously colored barks. The one here pho- 

 tographed was clinging to a birch tree, and the 

 likeness of color in the illustration is not the 

 mere sameness of printer's ink, but is a real 

 similarity. 



How is this change of color made? The proc- 

 ess has been carefully studied by Biedermann 

 in the case of the European tree-frog, and his 

 conclusions apply to the American species as 

 well. Gadow summarizes the explanation of 

 the mechanism as follows : 



" If we examine the green skin of the com- 

 mon tree-frog, Hyla arborea, under a low-power 

 and direct light, we see a mosaic of green, polyg- 

 onal areas, separated by dark lines and inter- 

 rupted by the openings of the skin-glands. 

 Seen from below, the skin appears black. Under 

 a stronger power the black layer is seen to be 

 composed of anastomosing and ramified black 

 pigment-cells. When the light shines through 

 the skin appears yellow. The epidermis itself 

 is quite colorless. The mosaic-layer is com- 

 posed of polygonal interference-cells, each of 

 which consists of a basal half which is granular 



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