36 NATURAL SELECTION ul 
This great principle gives us a clue which we can follow out 
in the study of many recondite phenomena, and leads us to 
seek a meaning and a purpose of some definite character in 
minutie which we should otherwise be almost sure to pass 
over as insignificant or unimportant. 
Popular Theories of Colour in Animals 
The adaptation of the external colouring of animals to 
their conditions of life has long been recognised, and has been 
imputed either to an originally created specific peculiarity, 
or to the direct action of climate, soil, or food. Where the 
former explanation has been accepted it has completely 
checked inquiry, since we could never get any further than 
the fact of the adaptation. There was nothing more to be 
known about the matter. The second explanation was soon 
found to be quite inadequate to deal with all the varied 
phases of the phenomena, and to be contradicted by many 
well known facts. For example, wild rabbits are always of 
gray or brown tints well suited for concealment among grass 
and fern. But when these rabbits are domesticated, without 
any change of climate or food, they vary into white or black, 
and these varieties may be multiplied to any extent, forming 
white or black races. Exactly the same thing has occurred 
with pigeons; and in the case of rats and mice, the white 
variety has not been shown to be at all dependent on altera- 
tion of climate, food, or other external conditions. In many 
cases the wings of an insect not only assume the exact tint of 
the bark or leaf it is accustomed to rest on, but the form and 
veining of the leaf or the exact rugosity of the bark is 
imitated ; and these detailed modifications cannot be reason- 
ably imputed to climate or to food, since in many cases the 
species does not feed on the substance it resembles, and when 
it does, no reasonable connection can be shown to exist 
between the supposed cause and the effect produced. It was 
reserved for the theory of Natural Selection to solve all these 
problems, and many others which were not at first supposed 
to be directly connected with them. To make these latter 
intelligible, it will be necessary to give a sketch of the whole 
series of phenomena which may be classed under the head of 
useful or protective resemblances. 
