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 permanent variety.
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