v COLOURS OF ANIMALS 359 



metallic colours of humming-birds, are probably due to fine 

 striae. 



Colour a Normal Product of Organisation 

 This outline sketch of the nature of colour in the animal 

 world, however imperfect, will at least serve to show us how 

 numerous and varied are the causes which perpetually tend 

 to the production of colour in animal tissues. If we consider 

 that in order to produce white all the rays which fall upon an 

 object must be reflected in nearly the same proportions as 

 they exist in solar light whereas, if rays of any one or more 

 kinds are absorbed or neutralised, the resultant reflected light 

 will be coloured ; and that this colour may be infinitely 

 varied according to the proportions in which different rays are 

 reflected or absorbed we should expect that white would be, 

 as it really is, comparatively rare and exceptional in nature. 1 

 The same observation will apply to black, which arises from 

 the absorption of all the different rays. Many of the com- 

 plex substances which exist in animals and plants are subject 

 to changes of colour under the influence of light, heat, 

 or chemical change, and we know that chemical changes 

 are continually occurring during the physiological processes 

 which occur in the body during development and growth. 

 We also find that every external character is subject to 

 minute changes, which are generally perceptible to us in 

 closely allied species ; and we can therefore have no doubt 

 that the extension and thickness of the transparent lamellae, 

 and the fineness of the striae or rugosities of the integuments, 

 must be undergoing constant minute changes ; and these 

 changes will very frequently produce changes of colour. These 

 considerations render it probable that colour is a normal and 

 even necessary result of the complex structure of animals and 

 plants ; and that those parts of an organism which are under- 

 going continual development and adaptation to new conditions, 

 and are also continually subject to the action of light and heat, 

 will be the parts in which changes of colour will most fre- 

 quently appear. Now there is little doubt that the external 

 changes of animals and plants in adaptation to the environ- 



1 White is produced by the scattering of the various rays in all directions, 

 and is often caused by air-bubbles or transparent globules. See Poulton's 

 Colours of Animals, pp. 3-6. 



