PDF - Wallace Online

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.