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As this is a very
important point it is well to state the evidence on which the belief is
founded. The wild rock-pigeon is of a slaty-blue colour, the tail has a
dark band across the end, the wings have two black bands, and the outer
tail-feathers are edged with white at the base. No other wild pigeon in
the world has this combination of characters. Now in every one of the
domestic varieties, even the most extreme, all the above marks, even to
the white edging of the outer tail-feathers, are sometimes found
perfectly developed. When birds belonging to two distinct breeds are
crossed one or more times, neither of the parents being blue, or having
any of the above-named marks, the mongrel offspring are very apt to
acquire some of these characters. Mr. Darwin gives instances which he
observed himself. He crossed some white fantails with some black barbs,
and the mongrels were black, brown, or mottled. He also crossed a barb
with a spot, which is a white bird with a red tail and red spot on the
forehead, and the mongrel offspring were dusky and mottled.


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