v COLOURS OF ANIMALS 359 
metallic colours of humming-birds, are probably due to fine 
striz. 
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.+ 
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 lamella, 
and the fineness of the striz 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. 
