ct correlation with the length of beak), the size of the crop and
of the upper part of the oesophagus; the development and abortion of
the oil-gland; the number of the primary wing and caudal feathers; the
relative length of the wing and tail to each other and to the body;
the relative length of the leg and foot; the number of scutellae on
the toes, the development of skin between the toes, are all points of
structure which are variable. The period at which the perfect plumage is
acquired varies, as does the state of the down with which the nestling
birds are clothed when hatched. The shape and size of the eggs vary. The
manner of flight, and in some breeds the voice and disposition, differ
remarkably. Lastly, in certain breeds, the males and females have come
to differ in a slight degree from each other.

Altogether at least a score of pigeons might be chosen, which, if shown
to an ornithologist, and he were told that they were wild birds, would
certainly be ranked by him as well-defined species. Moreover, I do not
believe that any ornithologist would in this case place the English
carrier, the short-faced tumbler, the runt, the barb, pouter, and
fantail in the same genus; more especially as in each of these breeds
several truly-inherited sub-breeds, or species, as he would call them,
could be shown him.

Great as are the differences between the breeds of the pigeon, I am
fully convinced that the common opinion of naturalists is correct,
namely, that all are descended from the rock-pigeon (Columba livia),
including under this term several geographical races or sub-species,
which differ from each other in the most trifling respects. As several
of the reasons which have led me to this belief are in some degree
applicable in other cases, I will here briefly give them. If the several
breeds are not varieties, and have not proceeded from the rock-pigeon,
they must have descended from at least seven or eight aboriginal stocks;
for it is impossible to make the present domestic breeds by the crossing
of any lesser number: how, for instance, could a pouter be produced
by crossing two breeds unless one of the parent-stocks possessed the
characteristic enormous crop? The supposed aboriginal stocks must all
have been rock-pigeons, that is, they did not breed or willingly perch
on trees. But besides C. livia, with its geographical sub-species, only
two or three other species of rock-pigeons are known; and these have
not any of the