a character derived from this cross,
after having disappeared during one or several generations, suddenly
reappears. A third class, differing only in the manner of reproduction,
might be formed to include all cases of reversion effected by means of
buds, and therefore independent of true or seminal generation. Perhaps even
a fourth class might be instituted, to include reversions by segments in
the same individual flower or fruit, and in different parts of the body in
the same individual animal as it grows old. But the two first main classes
will be sufficient for our purpose.

* * * * *

_Reversion to lost Characters by pure or uncrossed forms._--Striking
instances of this first class of cases were given in the sixth chapter,
namely, of the occasional reappearance, in variously-coloured pure breeds
of the pigeon, of blue birds with all the marks which characterise the wild
_Columba livia_. Similar cases were given in the case of the fowl. With the
common ass, as we now know that the legs of the wild progenitor are
striped, we may feel assured that the occasional appearance of such stripes
in the domestic animal is a case of simple reversion. But I shall be
compelled to refer again to these cases, and therefore will here pass them
over.

The aboriginal species from which our domesticated cattle and sheep are
descended, no doubt possessed horns; but several hornless breeds are now
well established. Yet in these--for instance, {30} in Southdown sheep--"it
is not unusual to find among the male lambs some with small horns." The
horns, which thus occasionally reappear in other polled breeds, either
"grow to the full size, or are curiously attached to the skin alone and
hang loosely down, or drop off."[66] The Galloways and Suffolk cattle have
been hornless for the last 100 or 150 years, but a horned calf, with the
horn often loosely attached, is occasionally born.[67]

There is reason to believe that sheep in their early domesticated condition
were "brown or dingy black;" but even in the time of David certain flocks
were spoken of as white as snow. During the classical period the sheep of
Spain are described by several ancient authors as being black, red, or
tawny.[68] At the present day, notwithstanding the great care which is
taken to prevent it, particoloured lambs and some entirely black are
occasionally dropped by our most highly improved and valued breeds, such as
the Southdowns. Since