The snake on the ground, coloured like the ground and shaped like the dead curved sticks or vines seen everywhere on the ground, and motionless like the vine, does not more closely assimilate to its surroundings than birds in trees often do--the birds which the Indian must also see. A stranger in these regions, even the naturalist with a sight quickened by enthusiasm, finds it hard to detect a parrot in a lofty tree, even when he knows that parrots are there; for their greenness in the green foliage and the correlated habit they possess of remaining silent and motionless in the presence of an intruder, make them invisible to him, and he is astonished that the Indian should be able to detect them. The Indian knows how to look for them; it is his trade, which is long to learn; but he is obliged to learn it, for his success in life, and even life itself, depends on it, since in the savage state Nature kills those who fail in her competitive examinations. The reader has doubtless often seen those little picture-puzzles, variously labelled "Where's the Cat?" or "Mad Bull," or "Burglar," or "Policeman," or "Snake in the Grass," etc., in which the thing named and to be discovered is formed by branches and foliage and by running water and drapery and lights and shadows in the sketch. At first one finds it extremely difficult to detect this picture within a picture; and at last, with the suddenness with which one invariably detects a dull-coloured snake, seen previously but not distinguished--the object sought for appears, and is thereafter so plain to the eye that one cannot look at the sketch, even held at a distance, without seeing the cat or policeman, or whatever it happens to be. And after patiently studying some scores or hundreds of these puzzles one gets to know just how to find the thing concealed, and finds it quickly--almost at a glance at last. Now, the ingenious person that first invented this pretty puzzle probably had no thought of Nature, with her curious imitative and protective resemblances, in his mind; yet he might very well have taken the hint from Nature, for this is what she does. The animal that must be seen to be avoided, and the animal that must be seen to be taken, are there in her picture, sketched in with such cunning art that to the uninstructed eye they form only portions of branch and foliage and shadow and sunlight above, and dull-hued or variegated earth and stones and dead and withering herbage underneath.