186 TROPICAL NATURE, AND OTHER ESSAYS. 



tions 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. The same 

 observation will apply to black, which arises from the 

 absorption of all the different rays. Many of the 

 complex 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 of 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 pro- 

 duce 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 frequently appear. Now there is little 

 doubt that the external changes of animals and plants in 

 adaptation to the environment are much more numerous 

 than the internal changes ; as seen in the varied character 

 of the integuments and appendages of animals — hair, 

 horns, scales, feathers, &c. &c. — and in plants, the leaves, 

 bark, flowers, and fruit, with their various modifications 

 - — as compared with the great uniformity in the texture 



