common in the wild _C.
livia_, in dovecot-pigeons, and in all the most highly modified races.
Thus, in all, the croup varies from white to {197} blue, being most
frequently white in Europe, and very generally blue in India.[340] We have
seen that the wild _C. livia_ in Europe, and dovecots in all parts of the
world, often have the upper wing-coverts chequered with black; and all the
most distinct races, when blue, are occasionally chequered in precisely the
same manner. Thus I have seen Pouters, Fantails, Carriers, Turbits,
Tumblers (Indian and English), Swallows, Bald-pates, and other toy-pigeons
blue and chequered; and Mr. Esquilant has seen a chequered Runt. I bred
from two pure blue Tumblers a chequered bird.

* * * * *

The facts hitherto given refer to the occasional appearance in pure races
of blue birds with black wing-bars, and likewise of blue and chequered
birds; but it will now be seen that when two birds belonging to distinct
races are crossed, neither of which have, nor probably have had during many
generations, a trace of blue in their plumage, or a trace of wing-bars and
the other characteristic marks, they very frequently produce mongrel
offspring of a blue colour, sometimes chequered, with black wing-bars, &c.;
or if not of a blue colour, yet with the several characteristic marks more
or less plainly developed. I was led to investigate this subject from MM.
Boitard and Corbie[341] having asserted that from crosses between certain
breeds it is rare to get anything but bisets or dovecot-pigeons, which, as
we know, are blue birds with the usual characteristic marks. We shall
hereafter see that this subject possesses, independently of our present
object, considerable interest, so that I will give the results of my own
trials in full. I selected for experiment races which, when pure, very
seldom produce birds of a blue colour, or have bars on their wings and
tail.

The nun is white, with the head, tail, and primary wing-feathers black; it
is a breed which was established as long ago {198} as the year 1600. I
crossed a male nun with a female red common tumbler, which latter variety
generally breeds true. Thus neither parent had a trace of blue in the
plumage, or of bars on the wing and tail. I should premise that common
tumblers are rarely blue in England. From the above cross I reared several
young: one was red over the whole back, but with the tail as blue as that
of the r