But this is merely
due to the wide variability of the degree of fissure and is to be
considered only as a fluctuation between somewhat widely distant
extremes, which may even apparently include [180] the form of the common
beech-leaves. It is not a bud-variation at all, and it is to be met with
quite commonly while the true reversions by buds are very rare and are
of the nature of sports appearing suddenly and remaining constant on the
same twig. Analogous phenomena of wide variability with true reversion
may be seen in the variety of the European hornbeam called _Carpinus
Betulus heterophylla_. The leaves of this tree generally show the
greatest diversity in form. Some other cases have been brought together
by Darwin. In the first place a subvariety of the weeping-willow with
leaves rolled up into a spiral coil. A tree of this kind kept true for
twenty-five years and then threw out a single upright shoot bearing flat
leaves. The barberry (_Berberis_) offers another case; it has a well
known variety with seedless fruit, which can be propagated by cuttings
or layers, but its runners are said always to revert to the common form,
and to produce ordinary berries with seeds. Most of the cases referred
to by Darwin, however, seem to be doubtful and cannot be considered as
true proofs of atavism until more is known about the circumstances under
which they were produced.
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