Many similar cases of analogous variation have been observed by Naudin in
the great gourd family, and by various authors in our cereals. Similar
cases occurring with insects under natural conditions have lately been
discussed with much ability by Mr. Walsh, who has grouped them under his
law of equable variability.
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, white loins, 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, probably for hundreds of generations. But when
a breed has been crossed only once by some other breed, the offspring
occasionally show for many generations a tendency to revert in character to
the foreign breed--some say, for a dozen or even a score of generations.
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