allas, believe that
crossing gives a strong tendency to variation, independently of the
characters inherited from either parent. They believe that it would be
easier to raise a pouter or fantail pigeon from crossing two distinct
species, neither of which possessed the characters of these races, than
from any single species. I can find few facts in support of this doctrine,
and believe in it only to a limited degree; but in a future chapter I shall
have to recur to this subject. For our present purpose the point is not
material. The question which concerns us is, whether or not many new and
important characters have arisen since man first domesticated the pigeon.
On the ordinary view, variability is due to changed conditions of life; on
the Pallasian doctrine, variability, or the appearance of new characters,
is due to some mysterious effect from the crossing of two species, neither
of which possess the characters in question. In some few instances it is
credible, though for several reasons not probable, that well-marked races
have been formed by crossing; for instance, a barb might perhaps have been
formed by a cross between a long-beaked carrier, having large eye-wattles,
and some short-beaked pigeon. That many races have been in some degree
modified by crossing, and that certain varieties which are distinguished
only by peculiar tints have arisen from crosses between
differently-coloured {189} varieties, may be admitted as almost certain. On
the doctrine, therefore, that the chief races owe their differences to
their descent from distinct species, we must admit that at least eight or
nine, or more probably a dozen species, all having the same habit of
breeding and roosting on rocks and living in society, either now exist
somewhere, or formerly existed but have become extinct as wild birds.
Considering how carefully wild pigeons have been collected throughout the
world, and what conspicuous birds they are, especially when frequenting
rocks, it is extremely improbable that eight or nine species, which were
long ago domesticated and therefore must have inhabited some anciently
known country, should still exist in the wild state and be unknown to
ornithologists.

The hypothesis that such species formerly existed, but have become extinct,
is in some slight degree more probable. But the extinction of so many
species within the historical period is a bold hypothesis, seeing how
little influence man has had in exterminating th