360 TROPICAL NATURE v 
ment are much more numerous than the internal changes ; as 
seen in the varied character of the integuments and append- 
ages of animals—hair, horns, scales, feathers, etc., etc.—and 
in plants, the leaves, bark, flowers, and fruit, with their 
various modifications—as compared with the great uniformity 
in the texture and composition of their internal tissues ; and 
this accords with the uniformity of the tints of blood, muscle, 
nerve, and bone throughout extensive groups, as compared 
with the great diversity of colour of their external organs. 
It seems a fair conclusion that colour per se may be considered 
to be normal, and to need no special accounting for; while 
the absence of colour (that is, either white or black), or the 
prevalence of certain colours to the constant exclusion of 
others, must be traced, like other modifications in the 
economy of living things, to the needs of the species. Or, 
looking at it in another aspect, we may say that amid the 
constant variations of animals and plants colour is ever tend- 
ing to vary and to appear where it is absent; and that natural 
selection is constantly eliminating such tints as are injurious 
to the species, or preserving and intensifying such as are 
useful. 
This view is in accordance with the well-known fact of 
colours which rarely or never appear in the species in a 
state of nature, continually occurring among domesticated 
animals and cultivated plants; showing us that the capacity 
to develop colour is ever present, so that almost any required 
tint can be produced which may, under changed conditions, 
be useful, in however small a degree. 
Let us now see how these principles will enable us to 
understand and explain the varied phenomena of colour in 
nature, taking them in the order of our functional classifica- 
tion of colours. 
Theory of Protective Colours 
We have seen that obscure or protective tints in their 
infinitely varied degrees are present in every part of the 
animal kingdom ; whole families or genera being often thus 
coloured. Now the various brown, earthy, ashy, and other 
neutral tints are those which would be most readily produced, 
because they are due to an irregular mixture of many kinds 
