PDF - Wallace Online
PDF - Wallace Online PDF - Wallace Online
358 TROPICAL NATUREand Mr. Sorby has detected no less than seven distinctcolouring matters in birds' eggs,several of which are chemicallyrelated to those of blood and bile. The same coloursare often produced by quite different substances in differentgroups, as shown by the red of the wing on the burnet-mothchanging to yellow with muriatic acid, while the red of thered-admiral butterfly undergoes no such change.These pigmental colours have a different character inanimals according to their positionin the integument. FollowingDr. Hagen's classification, epidermal colours are thosewhich exist in the external chitinised skin of insects, in thehairs of mammals, and, partially,in the feathers of birds.They are often very deep and rich, and do not fade afterdeath. The hypodermal colours are those which are situatedin the inferior soft layer of the skin. These are often oflighter and more vivid tints, and usually fade after death.Many of the reds and yellows of butterflies and birds belongto this class, as well as the intensely vivid hues of the nakedskin about the heads of many birds. These pigments sometimesexude through the pores, forming an evanescent bloomon the surface.Interference colours 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-bubbleand in thin films of oil on water or ; by fine striae which producecolours either by reflected or transmitted light, as seenin mother-of-pearl and in finely-ruled metallic surfaces. Inboth cases colour isproduced by light of one wave-lengthbeing neutralised, owing to one set of such waves beingretarded or shifted so as to be half a wave-length behind theother set, asmay be found explained in any treatise onphysical optics. The result is, that the complementary colourof that neutralised is seen ; and, as the thickness of the film orthe fineness of the striae undergo slight changes, almost anycolour can be produced. This is believed to be the originofmany of the glossy or metallic tints of insects, as well asthose of the feathers of some birds. The iridescent colours ofthe wings of dragon-flies are caused by the superposition oftwo or more transparent lamellae ;while the shining blue ofthe purple-emperor and other butterflies, and the intensely
v COLOURS OF ANIMALS 359metallic colours of humming-birds, are probablystriae.due to fineColour a Normal Product of OrganisationThis outline sketch of the nature of colour in the animalworld, however imperfect, will at least serve to show us hownumerous and varied are the causes which perpetually tendto the production of colour in animal tissues. If we considerthat in order to produce white all the rays which fall upon anobject must be reflected in nearly the same proportions asthey exist in solar light whereas, if rays of any one or morekinds are absorbed or neutralised, the resultant reflected lightwill be coloured ;and that this colour may be infinitelyvaried according to the proportions in which different rays arereflected or absorbed we should expect that white would be,as it really is, comparatively rare and 1exceptional in nature.The same observation will apply to black, which arises fromthe absorption of all the different rays. Many of the complexsubstances which exist in animals and plants are subjectto changes of colour under the influence of light, heat,or chemical change, and we know that chemical changesare continually occurring during the physiological processeswhich occur in the body during development and growth.We also find that every external character is subject tominute changes, which are generally perceptible to us inclosely allied species and we can therefore have no doubt;that the extension and thickness of the transparent lamellae,and the fineness of the striae or rugosities of the integuments,must be undergoing constant minute changes ;and thesechanges will very frequently produce changes of colour. Theseconsiderations render it probable that colour is a normal andeven necessary result of the complex structure of animals andplants and that those;parts of an organism which are undergoingcontinual development and adaptation to new conditions,and are also continually subject to the action of light and heat,will be the parts in which changes of colour will most frequentlyappear. Now there is little doubt that the externalchanges of animals and plants in adaptation to the environ-1White is produced by the scattering of the various rays in all directions,and is often caused by air-bubbles or transparent globules. See Poulton'sColours of Animals, pp. 3-6.
- Page 324 and 325: TROPICAL NATUREacross the expanded
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- Page 330 and 331: 314 TROPICAL NATUREover the lower a
- Page 332 and 333: 316 TROPICAL NATUREhumming-birds ma
- Page 334 and 335: 318 TROPICAL NATUREsweets, in all t
- Page 336 and 337: TROPICAL NATUREDisplay of Ornaments
- Page 338 and 339: 322 TROPICAL NATUREtamed a number o
- Page 340 and 341: TROPICAL NATUREallied species on Ch
- Page 342 and 343: TROPICAL NATUREvariation, and with
- Page 344 and 345: TROPICAL NATUREhalf completed.In th
- Page 346 and 347: TROPICAL NATUREdevelopment; the for
- Page 348 and 349: TROPICAL NATUREsoft, while the tong
- Page 350 and 351: 334 TROPICAL NATUREtubular tongue o
- Page 352 and 353: TROPICAL NATURElittle white-eyes (Z
- Page 354 and 355: THE COLOURS OF ANIMALS AND SEXUAL S
- Page 356 and 357: 340 TROPICAL NATUREand emotions may
- Page 358 and 359: 342 TROPICAL NATUREIt is true that
- Page 360 and 361: 344 TROPICAL NATUREprotecting them
- Page 362 and 363: 346 TROPICAL NATUREThis caterpillar
- Page 364 and 365: 348 TROPICAL NATUREweed of these tw
- Page 366 and 367: 350 TROPICAL NATUREflies, so that i
- Page 368 and 369: 352 TROPICAL NATUREThe universal av
- Page 370 and 371: 354 TROPICAL NATUREMr. Darwin's Des
- Page 372 and 373: 356 TROPICAL NATUREexternal objects
- Page 376 and 377: TROPICAL NATUREment are much more n
- Page 378 and 379: 362 TROPICAL NATUREprotective, serv
- Page 380 and 381: TROPICAL NATUEEincreasing complexit
- Page 382 and 383: 366 TROPICAL NATUREmale to special
- Page 384 and 385: TROPICAL NATURErunning to its burro
- Page 386 and 387: 370 TROPICAL NATUREturkey, and the
- Page 388 and 389: 372 TROPICAL NATUREghost-moth may r
- Page 390 and 391: 374 TROPICAL NATUREwhite companions
- Page 392 and 393: 376 TROPICAL NATUREtion, and what m
- Page 394 and 395: 378 TROPICAL NATURENatural Selectio
- Page 396 and 397: TROPICAL NATUREaway with the rapidi
- Page 398 and 399: 382 TROPICAL NATUREhere, not only i
- Page 400 and 401: 384 TROPICAL NATUREyet, as none of
- Page 402 and 403: 386 TROPICAL NATURE vmetallic colou
- Page 404 and 405: 388 TROPICAL NATUREThe Andaman isla
- Page 406 and 407: 390 TROPICAL NATUREpum, while black
- Page 408 and 409: TROPICAL NATUREthe large brilliantl
- Page 410 and 411: 394 TROPICAL NATUREzones ;while, es
- Page 412 and 413: 396 TROPICAL NATUREthese differ gre
- Page 414 and 415: 398 TROPICAL NATUREvery minute and
- Page 416 and 417: 400 TROPICAL NATUREsmall brown seed
- Page 418 and 419: TROPICAL NATUREdetails of these ada
- Page 420 and 421: 404 TROPICAL NATUREby butterflies.
- Page 422 and 423: 406 TROPICAL NATUREThe seeds of a p
v COLOURS OF ANIMALS 359metallic colours of humming-birds, are probablystriae.due to fineColour a Normal Product of OrganisationThis outline sketch of the nature of colour in the animalworld, however imperfect, will at least serve to show us hownumerous and varied are the causes which perpetually tendto the production of colour in animal tissues. If we considerthat in order to produce white all the rays which fall upon anobject must be reflected in nearly the same proportions asthey exist in solar light whereas, if rays of any one or morekinds are absorbed or neutralised, the resultant reflected lightwill be coloured ;and that this colour may be infinitelyvaried according to the proportions in which different rays arereflected or absorbed we should expect that white would be,as it really is, comparatively rare and 1exceptional in nature.The same observation will apply to black, which arises fromthe absorption of all the different rays. Many of the complexsubstances which exist in animals and plants are subjectto changes of colour under the influence of light, heat,or chemical change, and we know that chemical changesare continually occurring during the physiological processeswhich occur in the body during development and growth.We also find that every external character is subject tominute changes, which are generally perceptible to us inclosely allied species and we can therefore have no doubt;that the extension and thickness of the transparent lamellae,and the fineness of the striae or rugosities of the integuments,must be undergoing constant minute changes ;and thesechanges will very frequently produce changes of colour. Theseconsiderations render it probable that colour is a normal andeven necessary result of the complex structure of animals andplants and that those;parts of an organism which are undergoingcontinual development and adaptation to new conditions,and are also continually subject to the action of light and heat,will be the parts in which changes of colour will most frequentlyappear. Now there is little doubt that the externalchanges of animals and plants in adaptation to the environ-1White is produced by the scattering of the various rays in all directions,and is often caused by air-bubbles or transparent globules. See Poulton'sColours of Animals, pp. 3-6.
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