v COLOURS OF ANIMALS 393 



abnormal excrescence and ga'in additional vitality by the 

 adoption of the views here imperfectly set forth. 1 



Although we have arrived at the conclusion that tropical 

 light and heat can in no sense be considered as the cause of 

 colour, there remains to be explained the undoubted fact that 

 all the more intense and gorgeous tints are manifested by the 

 animal life of the tropics; while in some groups, such as 

 butterflies and birds, there is a marked preponderance of 

 highly-coloured species. This is probably due to a variety of 

 causes, some of which we can indicate, while others remain 

 to be discovered. The luxuriant vegetation of the tropics 

 throughout the entire year affords so much concealment that 

 colour may there be safely developed to a much greater 

 extent 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. Equally 

 important, probably, has been the permanence of favourable 

 conditions in the tropics, allowing certain groups to continue 

 dominant for long periods, and thus to carry out in one 

 unbroken line whatever developments of plumage or colour 

 may once have acquired an ascendency. Changes of climatal 

 conditions, and pre-eminently the glacial epoch, probably led 

 to the extinction of a host of highly-developed and finely- 

 coloured insects and birds in temperate zones, just as we 

 know that it led to the extinction of the larger and more 

 powerful mammalia which formerly characterised the tem- 

 perate zone in both hemispheres ; and this view is supported 

 by the fact that it is amongst those groups only which are 

 now exclusively tropical that all the more extraordinary 

 developments of ornament and colour are found. The obscure 

 local causes of colour to which we have referred will also 

 have acted most efficiently in regions where the climatal 

 condition remained constant, and where migration was unneces- 

 sary ; while whatever direct effect may be produced by light 

 or heat will necessarily have acted more powerfully within 

 the tropics. And lastly, all these causes have been in action 

 over an actually greater area in tropical than in temperate 



1 These views have been restated and enforced by much fresh illustration 

 and argument in Darwinism, chap. x. 



