probably be accounted for by {350} all the forms descended from the
same remote progenitor having a tendency to vary in the same manner.
Thus also we can perhaps understand the fact of some Laugher-pigeons
cooing almost like turtle-doves, and of several races having
peculiarities in their flight, for certain natural species (viz. _C.
torquatrix_ and _palumbus_) display singular vagaries in this respect.
In other cases a race, instead of imitating in character a distinct
species, resembles some other race; thus certain runts tremble and
slightly elevate their tails, like fantails; and turbits inflate the
upper part of their oesophagus, like pouter-pigeons.

It is a common circumstance to find certain coloured marks persistently
characterising all the species of a genus, but differing much in tint;
and the same thing occurs with the varieties of the pigeon: thus,
instead of the general plumage being blue with the wing-bars black,
there are snow-white varieties with red bars, and black varieties with
white bars; in other varieties the wing-bars, as we have seen, are
elegantly zoned with different tints. The Spot pigeon is characterised
by the whole plumage being white, excepting the tail and a spot on the
forehead; but these parts may be red, yellow, or black. In the
rock-pigeon and in many varieties the tail is blue, with the outer
edges of the outer feathers white; but in one sub-variety of the
monk-pigeon we have a reversed variation, for the tail is white, except
the outer edges of the outer feathers, which are black.[872]

With some species of birds, for instance with gulls, certain coloured
parts appear as if almost washed out, and I have observed exactly the
same appearance in the terminal dark tail-bar in certain pigeons, and
in the whole plumage of certain varieties of the duck. Analogous facts
in the vegetable kingdom could be given.

Many sub-varieties of the pigeon have reversed and somewhat lengthened
feathers on the back part of their heads, and this is certainly not due
to reversion to the parent-species, which shows no trace of such
structure; but when we remember that sub-varieties of the fowl, turkey,
canary-bird, duck, and goose, all have topknots or reversed feathers on
their heads; and when we remember that scarcely a single large natural
group of birds can be named