653 The Colors of Animals and Plants. | [November, 
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 complementary color of that neu- 
tralized is seen; and, as the thickness of the film or the fineness 
of the strie 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 la- 
mellæ; while the shining blue of the purple-emperor and other 
butterflies and the intensely metallic colors of humming-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 neutral- 
ized, the resultant reflected light will be colored, and that this 
color may be infinitely varied according to the proportions in 
which different rays are reflected or absorbed, we should expect 
that white would be, as it really is, comparatively rare and ex- 
ceptional 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 
chemical change, and we know that chemical changes are con- 
tinually occurring during the physiological processes of develop- 
ment and growth. We also find that every external character 15 
subject to minute changes, which are generally perceptible to us 
in closely allied species; and we can therefore have no 
that the extension and thickness of the transparent lamellae, and 
the fineness of the strie or rugosities of the integuments, must be 
undergoing constant minute changes ; and these changes will very 
frequently produce changes of color. These considerations 
it probable that color is a normal and even necessary Test” | 
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 continually subj ss 
the action of light and heat, will be the parts in which change 
doubt 
render 
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