THE COLORS OF ANIMALS AND PLANTS. 



531 



ferent substances in different groups, as shown 

 by the red of the wings of the burnet-moth chang- 

 ing to yellow with muriatic acid, while the red 

 of the red-admiral-butterfly undergoes no such 

 change. 



These pigmental colors have a different char- 

 acter in animals according to their position in the 

 integument. Following Dr. Hagen's classifica- 

 tion, epidermal colors are those which exist in 

 the external chitinized skin of insects, in the hairs 

 of mammals, and, partially, in the feathers of 

 birds. They are often very deep and rich, and 

 do not fade after death. The hypodermal colors 

 are those which are situated in the inferior soft 

 layer of the skin. These are often of lighter and 

 more vivid tints, and usually fade after death. 

 Many of the reds and yellows of butterflies and 

 birds belong to this class, as well as the intensely 

 vivid hues of the naked skin about the heads 

 of many birds. These colors sometimes exude 

 through the pores, forming an evanescent bloom 

 on the surface. 



Interference colors are less frequent in the 

 organic world. They are caused in two ways: 

 either by reflection from the two surfaces of 

 transparent films, as seen in the soap-bubble and 

 in thin films of oil on water ; or by fine striae, 

 which produce colors either by reflected or trans- 

 mitted light, as seen in mother-of-pearl, and in 

 finely-ruled metallic surfaces. In both cases col- 

 or is produced by light of one wave-length being 

 neutralized, owing to one set of such waves being 

 caused to be half a wave-length behind the other 

 set, as may be found explained in any treatise on 

 physical optics. The result is, that the comple- 

 mentary color of that neutralized is seen; and, 

 as the thickness of the film or the fineness of the 

 striae undergoes slight changes, almost any color 

 can be produced. This is believed to be the 

 origin of many of the glossy or metallic tints of 

 insects, as well as of those of the feathers of 

 some birds. The iridescent colors of the wings 

 of dragon-flies are caused by the superposition of 

 two or more transparent lamellae ; while the shin- 

 ing blue of the purple-emperor and other butter- 

 flies, and the intensely metallic colors of hum- 

 ming-birds, are probably due to fine striae. 



This outline sketch of the nature of color in 

 the animal world, however imperfect, will at least 

 serve to show us how numerous and varied are 

 the causes which perpetually tend to the produc- 

 tion of color in animal tissues. If we consider 

 that, in order to produce white, all the rays which 

 fall upon an object must be reflected in the same 

 proportions as they exist in solar light, whereas, 



if rays of any one or more kinds are absorbed or 

 neutralized, the resultant reflected light will be 

 colored, and that this color may be infinitely 

 varied according to the proportions in which dif- 

 ferent rays are reflected or absorbed, we should 

 expect that white would be, as it really is, com- 

 paratively rare and exceptional in Nature. The 

 same observation will apply to black, which arises 

 from the absorption of all the different rays. 

 Many of the complex substances which exist in 

 animals and plants are subject to changes of 

 color under the influence of light, heat, or chemi- 

 cal change, and we know that chemical changes 

 are continually occurring during the physiological 

 processes of development and growth. We also 

 find that every external character is subject to 

 minute changes, which are generally perceptible 

 to us in closely allied species ; and we can there- 

 fore have no doubt that the extension and thick- 

 ness of the transparent lamellae, and the fineness 

 of the striae or rugosities of the integuments, 

 must be undergoing constant minute changes ; 

 and these changes will very frequently produce 

 changes of color. These considerations render it 

 probable that color is a normal and even neces- 

 sary result of the complex structure of animals 

 and plants, and that those parts of an organism 

 which are undergoing continual development and 

 adaptation to new conditions, and are also con- 

 tinually subject to the action of light and heat, 

 will be the parts in which changes of color will 

 most frequently appear. Now, there is little 

 doubt that the external changes of animals and 

 plants in adaptation to the environment are much 

 more numerous than the internal changes, as 

 seen in the varied character of the integuments 

 and appendages of animals — hair, horns, scales, 

 feathers, etc. — and in plants, the leaves, bark, 

 flowers, and fruit, with their various appendages 

 — compared with the comparative uniformity of 

 the texture and composition of their internal tis- 

 sues ; and this accords with the uniformity of 

 the tints of blood, muscle, nerve, and bone, 

 throughout extensive groups, as compared with 

 the great diversity of color of their external or- 

 gans. It seems a fair conclusion that color per 

 se may be considered to be normal, and to need 

 no special accounting for, while the absence of 

 color (that is, either white or black), or the preva- 

 lence of certain colors to the constant exclusion 

 of others, must be traced, like other modifications 

 in the economy of living things, to the needs of 

 the species. Or, looking at it in another aspect, 

 we may say that amid the constant variations of 

 animals and plants color is ever tending to vary 



