ared in the first and second generation, during the course of only three
or four years, a considerable number of young birds, more or less plainly
coloured blue, and with most of the characteristic marks. When black and
white, or black and red birds, are crossed, it would appear that a slight
tendency exists in both parents to produce blue offspring, and that this,
when combined, overpowers the separate tendency in either parent to produce
black, or white, or red offspring.

If we reject the belief that all the races of the pigeon are the modified
descendants of _C. livia_, and suppose that they are descended from several
aboriginal stocks, then we must choose between the three following
assumptions: firstly, that at least eight or nine species formerly existed
which were aboriginally coloured in various ways, but have since varied in
so exactly the same manner as to assume the colouring of _C. livia_; but
this assumption throws not the least light on the appearance of such
colours and marks when the races are crossed. Or secondly, we may assume
that the aboriginal species {202} were all coloured blue, and had the
wing-bars and other characteristic marks of _C. livia_,--a supposition
which is highly improbable, as besides this one species no existing member
of the Columbidae presents these combined characters; and it would not be
possible to find any other instance of several species identical in
plumage, yet as different in important points of structure as are pouters,
fantails, carriers, tumblers, &c. Or lastly, we may assume that all the
races, whether descended from _C. livia_ or from several aboriginal
species, although they have been bred with so much care and are so highly
valued by fanciers, have all been crossed within a dozen or score of
generations with _C. livia_, and have thus acquired their tendency to
produce blue birds with the several characteristic marks. I have said that
it must be assumed that each race has been crossed with _C. livia_ within a
dozen, or, at the utmost, within a score of generations; for there is no
reason to believe that crossed offspring ever revert to one of their
ancestors when removed by a greater number of generations. In a breed which
has been crossed only once, the tendency to reversion will naturally become
less and less in the succeeding generations, as in each there will be less
and less of the blood of the foreign breed; but when there has been no
cross with a distinct bree