But what here more particularly concerns us is, that those
points in our domestic animals, which at the present time are undergoing
rapid change by continued selection, are also eminently liable to
variation. Look at the individuals of the same breed of the pigeon; and
see what a prodigious amount of difference there is in the beak of
tumblers, in the beak and wattle of carriers, in the carriage and tail of
fantails, etc., these being the points now mainly attended to by English
fanciers. Even in the same sub-breed, as in that of the short-faced
tumbler, it is notoriously difficult to breed nearly perfect birds, many
departing widely from the standard. There may truly be said to be a
constant struggle going on between, on the one hand, the tendency to
reversion to a less perfect state, as well as an innate tendency to new
variations, and, on the other hand, the power of steady selection to keep
the breed true. In the long run selection gains the day, and we do not
expect to fail so completely as to breed a bird as coarse as a common
tumbler pigeon from a good short-faced strain. But as long as selection is
rapidly going on, much variability in the parts undergoing modification may
always be expected.
Now let us turn to nature. When a part has been developed in an
extraordinary manner in any one species, compared with the other species of
the same genus, we may conclude that this part has undergone an
extraordinary amount of modification since the period when the several
species branched off from the common progenitor of the genus.
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