COLOURS OF ANIMALS. 161 



species, both of animals and plants, are each distinguished 

 by peculiar tints which vary very little, while the 

 minutest markings are often constant in thousands or 

 millions of individuals. All our field buttercups are 

 invariably yellow, and our poppies red ; while many of 

 our butterflies and birds resemble each other in every 

 spot and streak of colour through thousands of indivi- 

 duals. We also find that colour is constant in whole 

 genera and other groups of species. The Genistas are 

 all yellow, the Erythrinas all red ; many genera of Cara- 

 bidse are entirely black ; whole families of birds as the 

 Dendrocolaptidse are brown ; while among butterflies 

 the numerous species of Lycsena are all more or less blue, 

 those of Pontia white, and those of Callidryas yellow. 

 An extensive survey of the organic world thus leads us 

 to the conclusion that colour is by no means so unim- 

 portant or inconstant a character as at first sight it 

 appears to be ; and the more we examine it the more 

 convinced we shall become that it must serve some 

 purpose in nature, and that, besides charming us by its 

 diversity and beauty, it must be well worthy of our 

 attentive study, and have many secrets to unfold 

 to us. 



Theory of Heat and Light as producing Colour. In 

 commencing our study of the great mass of facts relating 

 to the colours of the organic world, it will be necessary 

 to consider first, how far the chief theories already 

 proposed will account for them. One of the most 

 obvious and most popular of these theories, and one 

 which is still held, in part at least, by many eminent 

 naturalists, is that colour is due to some direct action of 

 the heat and light of the sun thus at once accounting 



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