THE LAW WHICH UNDERLIES PROTECTIVE COLORATIOK^ 



By Abbott H. Thayer. 



This article is inteuded to set forth a beautiful law of nature which, 

 so far as I can discover, has uever been pointed out in print. It is the 

 law of gradation in the coloring of animals, and is responsible for most 

 of the phenomena of protective coloration except those properly called 

 mimicry. 



Naturalists have long recognized the fact that the coloring of many 

 animals makes them difficult to distinguish, and have called tlie whole 

 phenomenon protective coloration, little guessing how wonderful a fact 

 lay hidden under the name. 



Mimicry makes an animal appear to be some other thing, whereas 

 this newly discovered law makes him cease to appear to exist at all. 

 The following are some examples of true mimicry. The screech owl, 

 when startled, makes himself tall and slim, and with eyes shut to a 

 narrow line simulates a dead stub of the tree on which he sits. Certain 

 herons stretch their necks straight upward, and with head and green 

 beak pointed at the zenith pass themselves off for blades of sedge grass. 

 Certain harmless snakes spread their heads out flat, in imitation of 

 their poisonous cousins, and rattle with their tails in the leaves. Many 

 butterflies have stone or bark colored under sides to their wings, which 

 make them look like a bit of bark or lichen when they sit still on a 

 stone or tree trunk with wings shut over their backs. 



The newly discovered law may be stated thus : Animals are painted 

 by nature darkest on those parts which tend to be most lighted by the 

 sky's light, and vice versa. 



The accompanying diagram illustrates this statement. Animals are 

 colored by nature as in A, the sky lights them as in B, and the two 

 effects cancel each, other as in C. The result is that their gradation of 

 light and shade, by which opaque solid objects manifest themselves to 

 the eye, is effaced at every point, the cancellation being as complete at 

 one point as another, as in C of the diagram, and the spectator seems 

 to see right through the space really occupied by an opaque animal. 



1 Printed in The Ank, Vol. XIII, April and October, 1896. 



477 



