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COOK 



as there are a few bright colored desert animals. The pigments 

 which determine the color lie in the deeper layers of the skin, 

 and are readily concealed by a thickening of the superficial 

 layers, or by the development of darker pigments above to pro- 

 tect the lower cells from sunlight, as in the human species. 

 When the color is resident in an outer covering of hairs, feathers, 

 or scales, a very direct environmental reaction takes place, for 

 these are no longer actively living, and the strong sunlight can 

 bleach out the colors as well while the animals are alive as after 

 they are dead. This is true of many insects and also of the 

 horned toad, young or recently moulted individuals showing a 

 bright yellow which is lacking in the old. 



Finally, the protective coloration doctrine loses another instal- 

 ment in the fact that in the brilliant lights of deserts no colors 

 are very conspicuous. There is no occasion, so to speak, for 

 the development in desert animals of the brilliant tints which 

 may enable the members of the same species to more quickly 

 recognize each other in the sombre depths of tropical forests. 



There have been, no doubt, many cases where the protective 

 colors have been of immense advantage in the severe struggle 

 for existence to which animals are often exposed. Selection 

 must have had an immense influence in perfecting the marvel- 

 lous adjustments which many species have with their environ- 

 mental conditions. The nicety of some of . these adjustments 

 cannot be exaggerated — it is already past credence. A little 

 fish, common in Liberia, is so exactly the color of the water- 

 covered sandy stream-beds over which it swims that its presence 

 is often betrayed only by the darting shadows. A little frog 

 living in the sandy pools of the California desert canyons has 

 the same elaborately speckled browns and grays, and likewise 

 becomes invisible, except for the shadows. A slender pale gray 

 lizard of the Colorado desert of southern California even excels 

 the fish and the frog, for it seems to have the instinct of always 

 facing the sun when it stands upon a stone to gain a lookout. 

 In this position both its color and its shadow coincide with those 

 of the stone, and the concealment is perfect. 



The subject is one of tempting interest of detail, but enough 

 has been said, perhaps, to make it evident that the dull colora- 



