With pigeons, however, we have another case, namely, the occasional
appearance in all the breeds, of slaty-blue birds with two black bars
on the wings, a white rump, a bar at the end of the tail, with the
outer feathers externally edged near their bases with white. As all
these marks are characteristic of the parent rock-pigeon, I presume
that no one will doubt that this is a case of reversion, and not of a
new yet analogous variation appearing in the several breeds. We may I
think confidently come to this conclusion, because, as we have seen,
these coloured marks are eminently liable 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.
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