offspring from dovecotes and various other breeds are "generally very
fertile and hardy birds:" so again, MM. Boitard and Corbie,[281] after
forty-five years' experience, recommend persons to cross their breeds
for amusement; for, if they fail to make interesting birds, they will
succeed under an economical point of view, "as it is found that
mongrels are more fertile than pigeons of pure race."

I will refer only to one other animal, namely, the Hive-bee, because a
distinguished entomologist has advanced this as a case of inevitable
close interbreeding. As the hive is tenanted by a single female, it
might have been thought that her male and female offspring would always
have bred together, more especially as bees of different hives are
hostile to each other; a strange worker being almost always attacked
when trying to enter another hive. But Mr. Tegetmeier has shown[282]
that this instinct does not apply to drones, which are permitted to
enter any hive; so that there is no _a priori_ improbability of a queen
receiving a foreign drone. The fact of the union invariably and
necessarily taking place on the wing, during the queen's nuptial
flight, seems to be a special provision against continued
interbreeding. However this may be, experience has shown, since the
introduction of the yellow-banded Ligurian race into Germany and
England, that bees freely cross: Mr. Woodbury, who introduced Ligurian
bees into Devonshire, found during a single season that three stocks,
at distances of from one to two miles from his hives, were crossed by
his drones. In one case the Ligurian drones must have flown over the
city of Exeter, and over several intermediate hives. On another
occasion several common black queens were crossed by Ligurian drones at
a distance of from one to three and a half miles.[283]

{127}

_Plants._

When a single plant of a new species is introduced into any country, if
propagated by seed, many individuals will soon be raised, so that if
the proper insects be present there will be crossing. With
newly-introduced trees or other plants not propagated by seed we are
not here concerned. With old-established plants it is an almost
universal practice occasionally to make exchanges of seed, by which
means individuals which have been exposed to different conditions of
life,--and this