PDF - Wallace Online
PDF - Wallace Online PDF - Wallace Online
TROPICAL NATUREthe large brilliantly -marked wings of some butterflies andmoths.Colours are produced or intensified by processes of development,either where the integument or its appendages undergogreat extension or modification, or where there is a surplus ofvital energy, as in male animals generally, and more especiallyat the breeding season.Colours are also more or less influenced by a variety ofcauses, such as the nature of the food, the photographic orphysiological action of light, and also by some unknown localaction, probably dependent on chemical peculiarities in the soilor vegetation.These various causes have acted and reacted in a varietyof ways, and have been modified by conditions dependent onage or on sex, on competition with new forms, or on geographicalor climatic changes. In so complex a subject, forwhich experiment and systematic inquiry have done so little,we cannot expect to explain every individual case, or solveevery difficulty but it is believed that all the;great features ofanimal coloration and many of the details become explicableon the principles we have endeavoured to lay down.It will perhaps be considered presumptuous to put forththis sketch of the subject of colour in animals as a substitutefor one of Mr. Darwin's most highly elaborated theoriesthat of voluntary or perceptive sexual selection ; yet I ventureto think that it is more in accordance with the whole ofthe facts, and with the theory of natural selection itself ;andI would ask such ofmy readers as may be sufficiently interestedin the subject, to read again chapters xi. to xvi. ofthe Descent of Man, and consider the whole subject from thepoint of view here laid down. The explanation of almost allthe ornaments and colours of birds and insects as having beenproduced by the perceptions and choice of the females, has,I believe, staggered many evolutionists, but has been provisionallyaccepted because it was the only theory that evenattempted to explain the facts. It may perhaps be a reliefto some of them, as it has been to myself, to find that thephenomena can be conceived as dependent on the generallaws of development, and on the action of " natural selection,"which theory will, I venture to think, be relieved from an
v COLOURS OF ANIMALS 393abnormal excrescence and ga'in additional vitality by theadoption of the views here 1imperfectly set forth.Although we have arrived at the conclusion that tropicallight and heat can in no sense be considered as the cause ofcolour, there remains to be explained the undoubted fact thatall the more intense and gorgeous tints are manifested by theanimal life of the tropics; while in some groups, such asbutterflies and birds, there is a marked preponderance ofhighly-coloured species. This is probably due to a variety ofcauses, some of which we can indicate, while others remainto be discovered. The luxuriant vegetation of the tropicsthroughout the entire year affords so much concealment thatcolour may there be safely developed to a much greaterextent than in climates where the trees are bare in winter,during which season the struggle for existence is most severe,and even the slightest disadvantage may prove fatal. Equallyimportant, probably, has been the permanence of favourableconditions in the tropics, allowing certain groups to continuedominant for long periods, and thus to carry out in oneunbroken line whatever developments of plumage or colourmay once have acquired an ascendency. Changes of climatalconditions, and pre-eminently the glacial epoch, probably ledto the extinction of a host of highly-developed and finelycolouredinsects and birds in temperate zones, just as weknow that it led to the extinction of the larger and morepowerful mammalia which formerly characterised the temperatezone in both hemispheres and this view is ; supportedby the fact that it is amongst those groups only which arenow exclusively tropical that all the more extraordinarydevelopments of ornament and colour are found. The obscurelocal causes of colour to which we have referred will alsohave acted most efficiently in regions where the climatalcondition remained constant, and where migration was unnecessary;while whatever direct effect may be produced by lightor heat will necessarily have acted more powerfully withinthe tropics. And lastly,all these causes have been in actionover an actually greater area in tropical than in temperate1 These views have been restated and enforced by much fresh illustrationand argument in Darwinism, chap. x.
- Page 358 and 359: 342 TROPICAL NATUREIt is true that
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- Page 370 and 371: 354 TROPICAL NATUREMr. Darwin's Des
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- Page 374 and 375: 358 TROPICAL NATUREand Mr. Sorby ha
- Page 376 and 377: TROPICAL NATUREment are much more n
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- Page 402 and 403: 386 TROPICAL NATURE vmetallic colou
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- Page 406 and 407: 390 TROPICAL NATUREpum, while black
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- Page 412 and 413: 396 TROPICAL NATUREthese differ gre
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TROPICAL NATUREthe large brilliantly -marked wings of some butterflies andmoths.Colours are produced or intensified by processes of development,either where the integument or its appendages undergogreat extension or modification, or where there is a surplus ofvital energy, as in male animals generally, and more especiallyat the breeding season.Colours are also more or less influenced by a variety ofcauses, such as the nature of the food, the photographic orphysiological action of light, and also by some unknown localaction, probably dependent on chemical peculiarities in the soilor vegetation.These various causes have acted and reacted in a varietyof ways, and have been modified by conditions dependent onage or on sex, on competition with new forms, or on geographicalor climatic changes. In so complex a subject, forwhich experiment and systematic inquiry have done so little,we cannot expect to explain every individual case, or solveevery difficulty but it is believed that all the;great features ofanimal coloration and many of the details become explicableon the principles we have endeavoured to lay down.It will perhaps be considered presumptuous to put forththis sketch of the subject of colour in animals as a substitutefor one of Mr. Darwin's most highly elaborated theoriesthat of voluntary or perceptive sexual selection ; yet I ventureto think that it is more in accordance with the whole ofthe facts, and with the theory of natural selection itself ;andI would ask such ofmy readers as may be sufficiently interestedin the subject, to read again chapters xi. to xvi. ofthe Descent of Man, and consider the whole subject from thepoint of view here laid down. The explanation of almost allthe ornaments and colours of birds and insects as having beenproduced by the perceptions and choice of the females, has,I believe, staggered many evolutionists, but has been provisionallyaccepted because it was the only theory that evenattempted to explain the facts. It may perhaps be a reliefto some of them, as it has been to myself, to find that thephenomena can be conceived as dependent on the generallaws of development, and on the action of " natural selection,"which theory will, I venture to think, be relieved from an
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