354 TROPICAL NATURE 



Mr. Darwin's Descent of Man, chapters x. to xviii., and to 

 chapters iii. iv. and vii. of the first portion of the present 

 volume. 



Normal Colours 



The fourth group of normally coloured animals includes 

 all species which are brilliantly or conspicuously coloured in 

 both sexes, and for whose particular colours we can assign no 

 function or use. 1 It comprises an immense number of showy 

 birds, such as kingfishers, barbets, toucans, lories, tits, and 

 starlings ; among insects most of the largest and handsomest 

 butterflies, innumerable bright-coloured beetles, locusts, dragon- 

 flies, and hymenoptera ; a few mammalia, as the zebras ; a 

 great number of marine fishes ; thousands of striped and 

 spotted caterpillars ; and abundance of mollusca, star-fish, and 

 other marine animals. Among these we have included some 

 which, like the gaudy caterpillars, have warning colours ; but 

 as that theory does not explain the particular colours or the 

 varied patterns with which they are adorned, it is best to 

 include them also in this class. It is a suggestive fact that 

 all the brightly-coloured birds mentioned above build in holes 

 or form covered nests, so that the females do not need that 

 protection during the breeding season which I believe to be 

 one of the chief causes of the dull colour of female birds when 

 their partners are gaily coloured. This subject is fully argued 

 in chapter vi. of the present volume. 



Leaving the colours of flowers to be discussed in another 

 chapter, we will now consider how the general facts of colour 

 here sketched out can be explained. We have first to inquire 

 what is colour, and how it is produced j secondly, what is 

 known of the causes of change of colour ; and, lastly, what 

 theory best accords with the whole assemblage of facts. 



The Nature of Colour 



The sensation of colour is caused by vibrations or undula- 

 tions of the ethereal medium of different lengths and velocities. 

 The whole body of vibrations caused by the sun is termed 



* Distinctness of marking for purposes of recognition is probably the use 

 in all cases. See p. 367, and Darwinism, p. 217. 



