When, therefore, we
cannot see the meaning of the colour, we may suppose that it has been
protective in some ancestral form, and, not being hurtful, has persisted
under changed conditions which rendered the protection needless.
We may divide all eggs, for our present purpose, into two great
divisions; those which are white or nearly so, and those which are
distinctly coloured or spotted. Egg-shells being composed mainly of
carbonate of lime, we may assume that the primitive colour of birds'
eggs was white, a colour that prevails now among the other egg-bearing
vertebrates--lizards, crocodiles, turtles, and snakes; and we might,
therefore, expect that this colour would continue where its presence had
no disadvantages. Now, as a matter of fact, we find that in all the
groups of birds which lay their eggs in concealed places, whether in
holes of trees or in the ground, or in domed or covered nests, the eggs
are either pure white or of very pale uniform coloration. Such is the
case with kingfishers, bee-eaters, penguins, and puffins, which nest in
holes in the ground; with the great parrot family, the woodpeckers, the
rollers, hoopoes, trogons, owls, and some others, which build in holes
in trees or other concealed places; while martins, wrens,
willow-warblers, and Australian finches, build domed or covered nests,
and usually have white eggs.
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