TEE COLORS OF ANIMALS AND PLANTS. 



529 



series we have spots or patches of vivid color in r 

 the male which are represented in the female by 

 far less brilliant tints or are altogether wanting ; 

 as exemplied in the gold-crest warbler, the green 

 woodpecker, and most of the orange-tip butter- 

 flies {Anihocharis). Proceeding with our survey 

 we find greater and greater differences of color 

 in the sexes, till we arrive at such extreme cases 

 as some of the pheasants, the chatterers, tanagers, 

 and birds-of-paradise, in which the male is adorned 

 with the most gorgeous and vivid colors, while 

 the female is usually dull brown, or olive-green, 

 and often shows no approximation whatever to 

 the varied tints of her partner. Similar phenom- 

 ena occur among butterflies ; and in both these 

 classes there are also a considerable number of 

 cases in which both sexes are highly colored in 

 a different way. Thus many woodpeckers have 

 the head in the male red, in the female yellow ; 

 while some parrots have red spots in the male, 

 replaced by blue in the female, as in Psittacula 

 diopthalma. In many South American papilios 

 green spots on the male are represented by red 

 on the female ; and in several species of the 

 genus Epicalia orange bands in the male are 

 replaced by blue in the female, a similar change 

 of color as in the small parrot above referred to. 

 For fuller details of the varieties of sexual color- 

 ation we refer our readers to Mr. Darwin's " De- 

 scent of Man," chapters x. to xviii., and to chap- 

 ters iii., iv., and vii., of my " Contributions to the 

 Theory of Natural Selection." 



The fourth group — of typically-colored ani- 

 mals — includes all species which are brilliantly 

 or conspicuously colored in both sexes, and for 

 whose particular colors we can assign no func- 

 tion or use. It comprises an immense number 

 of showy birds, such as kingfishers, barbets, 

 toucans, lories, tits, and starlings ; among in- 

 sects most of the largest and handsomest butter- 

 flies, innumerable bright-colored beetles, locusts, 

 dragon-flies, and hymenoptera ; a few mammalia, 

 as the zebras ; a great number of marine fishes ; 

 thousands of striped and spotted caterpillars ; 

 and abundance of mollusca, starfish, and other 

 marine animals. Among these we have included 

 some which, like the gaudy caterpillars, have 

 warning colors ; but as that theory does not ex- 

 plain the particular colors or the varied patterns 

 with which they are adorned, it is best to include 

 them also in this class. It is a suggestive fact that 

 all the brightly-colored birds mentioned above 

 build in holes or form covered nests, so that the 

 females do not need that protection during the 

 breeding-season, which I believe to be one of the 



34 



chief causes of the dull color of female birds when 

 their partners are gayly colored. This subject is ful- 

 ly argued in my " Contributions," etc., chapter vii. 



As the colors of plants and flowers are very 

 different from those of animals, both in their dis- 

 tribution and functions, it will be well to treat 

 them separately: we will therefore now consider 

 how the general facts of color here sketched out 

 can be explained. We have first to inquire what 

 is color, and how it is produced ; what is known 

 of the causes of change of color ; and what theory 

 best accords with the whole assemblage of facts. 



The sensation of color is caused by vibrations 

 or undulations of the ethereal medium of different 

 lengths and velocities. The whole body of vibra- 

 tions caused by the sun is termed radiation, and 

 consists of sets of waves which vary considerably 

 in their dimensions and their rate of vibration, 

 but of which the middle portion only is capable 

 of exciting in us sensations of light and color. 

 Beginning with the largest and slowest rays or 

 wave-vibrations, we have first those which pro- 

 duce heat-sensations only ; as they get smaller 

 and quicker, we perceive a dull-red color ; and as 

 the waves increase in rapidity of vibration and 

 diminish in size, we get successively sensations 

 of orange, yellow, green, blue, indigo, and violet, 

 all fading imperceptibly into each other. Then 

 come more invisible rays, of shorter wave-length 

 and quicker vibration, which produce, solely or 

 chiefly, chemical effects. The red rays, which 

 first become visible, have been ascertained to vi- 

 brate at the rate of 458 millions of millions of 

 times in a second, the length of each wave being 

 369Tny of an inch ; while the violet rays, which 

 last remain visible, vibrate 72*7 millions of millions 

 of times per second, and have a wave-length of 

 tTTsTtr 0I * an i ncn - Although the waves vibrate 

 at different rates, they are all propagated through 

 the ether with the same velocity (192,000 miles 

 per second), just as different musical sounds, 

 which are produced by waves of air of different 

 lengths and rates of vibration, travel at the same 

 rate, so that a tune played several hundred yards 

 off reaches the ear in correct time. There are, 

 therefore, an almost infinite number of different 

 color-producing vibrations, and these may be 

 combined in an almost infinite variety of ways, 

 so as to excite in us the sensation of all the va- 

 ried colors and tints we are capable of perceiving. 

 When all the different kinds of rays reach us in 

 the proportion in which they exist in the light of 

 the sun, they produce the sensation of white. If 

 the rays which excite the sensation of any one 



