in the following classification to
rank them under Groups, Races, and Sub-races; to which varieties and
sub-varieties, all strictly inheriting their proper characters, must often
be added. Even with the individuals of the same sub-variety, when long kept
by different fanciers, different strains can sometimes be recognised. There
can be no doubt that, if well-characterized forms of the several Races had
been found wild, all would have been ranked as distinct species, and
several of them would certainly have been placed by ornithologists in
distinct genera. A good classification of the various domestic breeds is
extremely difficult, owing to the manner in which many of the forms
graduate into each other; but it is curious how exactly the same
difficulties are encountered, and the same rules have to be followed, as in
the classification of any natural but difficult group of organic beings. An
"artificial classification" might be followed which would present fewer
difficulties than a "natural classification;" but then it would interrupt
many plain affinities. Extreme forms can readily be defined; but
intermediate and troublesome forms {134} often destroy our definitions.
Forms which may be called "aberrant" must sometimes be included within
groups to which they do not accurately belong. Characters of all kinds must
be used; but as with birds in a state of nature, those afforded by the beak
are the best and most readily appreciated. It is not possible to weigh the
importance of all the characters which have to be used so as to make the
groups and sub-groups of equal value. Lastly, a group may contain only one
race, and another and less distinctly defined group may contain several
races and sub-races, and in this case it is difficult, as in the
classification of natural species, to avoid placing too high a value on
characters which are common to a large number of forms.

In my measurements I have never trusted to the eye; and when speaking of a
part being large or small, I always refer to the wild rock-pigeon (_Columba
livia_) as the standard of comparison. The measurements are given in
decimals of an inch.[279]

I will now give a brief description of all the principal breeds. The
following diagram may aid the reader in learning their names and seeing
their affinities. The rock-pigeon, or _Columba livia_ (including under this
name two or three closely-allied sub-species or geographical races,
hereafter to be described), may