COLOURS OF ANIMALS. 219 



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 per- 

 manence 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 con- 

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

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

 coloured insects and birds intemperate zones ; just as we 

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

 powerful mammalia which formerly characterised the 

 temperate 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 



