ly 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. After twelve generations, the proportion of blood, to use a
common expression, from 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 remnant 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, as was formerly remarked, for all that we
can see to the contrary, be 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 one individual suddenly takes after an ancestor removed by
some hundred generations, but that in each successive generation the
character in question has been lying latent, and at last, under unknown
favourable conditions, is developed. With the barb-pigeon, for instance,
which very rarely produces a blue bird, it is probable that there is a
latent tendency in each generation to produce blue plumage. The abstract
improbability of such a tendency being transmitted through a vast number
of generations, is not greater than that of quite useless or rudimentary
organs being similarly transmitted. A mere tendency to produce a
rudiment is indeed sometimes thus inherited.

As all the species of the same genus are supposed to be descended from
a common progenitor, it might be expected that they would occasionally
vary in an analogous manner; so that the varieties of two or more
species would resemble each other, or that a variety of one species
would resemble in certain characters another and distinct species,
this other species being, according to our view, only a well-marked and