not to the vera causa of community of descent, and a consequent
tendency to vary in a like manner, but to three separate yet closely
related acts of creation.
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.
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
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