or at least much less brightly colored than birds, etc. The fact that they 

 are so becomes in fact one more strong link in the chain of evidence in proof 

 of the universal and paramount importance of concealing-coloration. Mam- 

 mals (we will exclude the aberrant forms, as the bats — winged, nocturnal, 

 and cavern-haunting — and the marine types) are characteristically flightless, 

 and hence, in a great measure, tied to earth. In the forest, the outermost 

 skyward excursions of the most arboreal species rarely or never take them 

 into the gay regions inhabited by the more brilliant birds and butterflies. 

 For these are masters of that unstable element, the air, and can go whither 

 they please above as well as upon and through the ground and the forest. 

 Hovering, flitting, perching lightly, always ready to resort to their wings in 

 an instant if the perch should fail them, even many of the heavier members 

 of this gifted class — even many of the birds, in short — are wont to pass most 

 of their time in the brilliantly lighted outermost border of the forest, among 

 the very tips of the slenderest twigs, where most of the fruits and flowers grow, 

 but whither, even the most agile climbers of all the wingless mammals dare 

 not venture. Metaphorically speaking, birds and butterflies are creatures of 

 the grave and ponderous globe's exterior efilorescence, of the colored foam 

 at the outermost edge of things, of the borderland between earth and sky; 

 while mammals, man included, are citizens of the sober underworld. Plod- 

 ding, earth-bound things, they walk upon the solid ground, and drive their 

 tunnels in the darkness under it; while some of them ascend its skyward ex- 

 crescences, the trees; but high as they may go they are still the creatiures of 

 the 'underworld,' and the bright realm of butterflies and birds is still as a 

 rule above them. By their life-laws, they are forever associated in close con- 

 tiguity with things brown and gray and black;— mud^ sand, the dead leaves 

 and twigs of the forest floor, rocks and pebbles, tree trunks and branches — 

 and the black holes and shadows among all these things. Some few of the 

 most arboreal kinds live in the realm of preponderant sylvan greenness (see 

 p. io8 of Chapter XIX), and several of these (small monkeys, etc.) have olive- 

 green or even clear green and sometimes yellow fur. But as no wingless 



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