most significant, since, in almost every case, they accompany some important 

 peculiarity in the animal's habits or physical characteristics. In the cos- 

 tumes of many bats, for instance, the counter shading is defective, or alto- 

 gether lacking, the fur of the lower surface being almost or quite as dark as 

 that of the upper. But this is in strictest keeping with their habits, for they 

 are nocturnal and volant, flying swiftly, and for the most part feeding on the 

 wing, like goatsuckers, while, unlike goatsuckers, they sleep by day in the 

 pitchy darkness of deep caves and ruins, or in hollow trees, suspended by their 

 hind claws, and hanging head downward, perpendicularly. Hence, it ap- 

 pears. Nature has been very little concerned with giving them disguising 

 coloration. Their perpendicular sleeping-posture in itself precludes the pos- 

 sibility of their benefiting by the regulation obliterative shading while they 

 are at rest. If they are to be counter shaded at all, it must be from tail to 

 head, rather than from back to belly. Traces of such an aberrant shading 

 exist on many species, in the shape of white or yellowish markings on the 

 face,* and a brown paler than that of the rump and belly on the fore-back 

 and fore-breast. This partial counter shading, as well as bats' prevailing 

 earthy and rock-brown color, serves them in the cases where their roosts are 

 more or less exposed to the daylight. Some kinds, indeed, habitually roost 

 in the open air, under big tropical leaves, under the branches of trees, and 

 on their trunks. The tree-trunk species while roosting are lighted as are 

 scansorial birds, and for obliteration they would have to be counter shaded 

 in the normal way, — as some of them are. But several of these open-air bats 

 are developed for mimicry instead of obliteration. Thus a beautiful little 

 South American species is formed, marked and colored to look like a woody 

 knot or other excrescence on the underside of a mangrove branch, whereto it 

 clings, by day; not hanging downward, but pressed close against the bark, 

 holding on both with feet and finger-hooks. Usually several are found to- 

 gether, in a neatly distributed little group. Disturbed, they take wing all 

 together, with a tiny, complaining twitter, and fly away like a troop of sand- 



* See also Chapter XXII, p. 157. 

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