coloured objects on the
female bird during the period preceding incubation; and have expended
much ingenuity in suggesting the objects that may have caused the eggs
of one bird to be blue, another brown, and another pink.[82] But no
evidence has been presented to prove that any effects whatever are
produced by this cause, while there seems no difficulty in accounting
for the facts by individual variability and the action of natural
selection. The changes that occur in the conditions of existence of
birds must sometimes render the concealment less perfect than it may
once have been; and when any danger arises from this cause, it may be
met either by some change in the colour of the eggs, or in the
structure or position of the nest, or by the increased care which the
parents bestow upon the eggs. In this way the various divergences which
now so often puzzle us may have arisen.

_Colour as a Means of Recognition._

If we consider the habits and life-histories of those animals which are
more or less gregarious, comprising a large proportion of the herbivora,
some carnivora, and a considerable number of all orders of birds, we
shall see that a means of ready recognition of its own kind, at a
distance or during rapid motion, in the dusk of twilight or in partial
cover, must be of the greatest advantage and often lead to the
preservation of life. Animals of this kind will not usually receive a
stranger into their midst. While they keep together they are generally
safe from attack, but a solitary straggler becomes an easy prey to the
enemy; it is, therefore, of the highest importance that, in such a case,
the wanderer should have every facility for discovering its companions
with certainty at any distance within the range of vision.

Some means of easy recognition must be of vital importance to the young
and inexperienced of each flock, and it also enables the sexes to
recognise their kind and thus avoid the evils of infertile crosses; and
I am inclined to believe that its necessity has had a more widespread
influence in determining the diversities of animal coloration than any
other cause whatever. To it may probably be imputed the singular fact
that, whereas bilateral symmetry of coloration is very frequently lost
among domesticated animals, it almost universally prevails in a state of
nature; for if the two sides of an animal were unlike, and the diversity
of coloration among domestic animals occurred in a wild state, ea