most beautifully simple to be found among vertebrates, not excepting fishes. 

 There can be no finer display of this great principle in full and pure applica- 

 tion than is furnished by the common green Grass Snake (Liopeltis vemalis) 

 of northeastern North America. (See Fig. 121.) Deep grass-green along the 

 upper median line, his color shades lighter by imperceptible gradations down 

 his sides, passing through pale straw-green along the edges of his ventral 

 surface, and culminating in dim greenish white on the flat underside. The 

 effect of this exquisite shade-gradation is to make the little snake appear of a 

 uniform, intangible, grassy green, when he lies at rest or glides along, on the 

 ground or in the bushes, in his normal position. Turn him over, so that his 

 pale belly is uppermost, and he becomes very conspicuous — looking grossly 

 solid (Fig. 122). He is still, on the whole, green, like his surroundings — 

 but by no means 'obliterated.' (!) In a normal position, 'flattened out' 

 completely by the counter shading, the snake is not only likely to look merged 

 into his general green background of grass (or other vegetation), but, when 

 this illusion fails, and parts of his surface definitely catch the eye, they will as 

 a rule pass for portions of broad grass blades or flat leaf-surfaces. This is 

 mimicry, after a fashion, but it is mimicry achieved only with the aid of obliter- 

 ative shading — and the effect is decidedly of minor importance. Such im- 

 minglement of the two principles ('obliteration' and mimicry) occurs much 

 more pronouncedly among lepidopterous larvae, which we shall describe in a 

 later chapter. In the case of the little green snake, the intrusion of the semi- 

 mimetic effect is due to his complete lack of markings. Any markings, super- 

 imposed on so perfect a counter shading, would almost infallibly look as if 

 they belonged to the background, thus clinching the obliteration. But simple, 

 immaculate coloration, with full obliterative shading, is very common among 

 snakes. In fact, it is about the commonest, as it is the simplest, type of ophid- 

 ian coloration, both among terrestrial and arboreal species. Most of the 

 snakes thus colored are swift and active kinds that seek and chase their prey 

 instead of lying in wait for it. The gliding motion of their long, slim bodies 

 would often be revealed by markings, in cases where without them it might pass 



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