e to appear in the crossed offspring
of two distinct and differently coloured breeds; and in this case there
is nothing in the external conditions of life to cause the reappearance
of the slaty-blue, with the several marks, beyond the influence of the
mere act of crossing on the laws of inheritance.

No doubt it is a very surprising fact that characters should reappear
after having been lost for many, perhaps for hundreds of generations.
But when a breed has been crossed only once by some other breed, the
offspring occasionally show a tendency to revert in character to the
foreign breed for many generations--some say, for a dozen or even a
score of generations. After twelve generations, the proportion of blood,
to use a common expression, of any one ancestor, is only 1 in 2048; and
yet, as we see, it is generally believed that a tendency to reversion
is retained by this very small proportion of foreign blood. In a breed
which has not been crossed, but in which BOTH parents have lost some
character which their progenitor possessed, the tendency, whether strong
or weak, to reproduce the lost character might be, as was formerly
remarked, for all that we can see to the contrary, transmitted for
almost any number of generations. When a character which has been lost
in a breed, reappears after a great number of generations, the most
probable hypothesis is, not that the offspring suddenly takes after an
ancestor some hundred generations distant, but that in each successive
generation there has been a tendency to reproduce the character in
question, which at last, under unknown favourable conditions, gains an
ascendancy. For instance, it is probable that in each generation of the
barb-pigeon, which produces most rarely a blue and black-barred bird,
there has been a tendency in each generation in the plumage to assume
this colour. This view is hypothetical, but could be supported by some
facts; and I can see no more abstract improbability in a tendency
to produce any character being inherited for an endless number of
generations, than in quite useless or rudimentary organs being, as we
all know them to be, thus inherited. Indeed, we may sometimes observe
a mere tendency to produce a rudiment inherited: for instance, in the
common snapdragon (Antirrhinum) a rudiment of a fifth stamen so often
appears, that this plant must have an inherited tendency to produce it.

As all the species of the same genus are supposed, on my theory, t