, produce fewer varieties under
cultivation than the already variable species of larger genera.

Although we have not at present sufficient evidence that the crossing
of species, which have never been cultivated, leads to the appearance
of new characters, this apparently does occur with species which have
been already rendered in some degree variable through cultivation.
Hence crossing, like any other change in the conditions of life, seems
to be an element, probably a potent one, in causing variability. But we
seldom have the means of distinguishing, as previously remarked,
between the appearance of really new characters and the reappearance of
long-lost characters, evoked through the act of crossing. I will give
an instance of the difficulty in distinguishing such cases. The species
of Datura may be divided into two sections, those having white flowers
with green stems, and those having purple flowers with brown stems: now
Naudin[642] crossed _Datura laevis_ and _ferox_, both of which belong
to the white section, and raised from them 205 hybrids. Of these
hybrids, every one had brown stems and bore purple flowers; so that
they resembled the species of the other section of the genus, and not
their own two parents. Naudin was so much astonished at this fact, that
he was led carefully to observe both parent-species, and he discovered
that the pure seedlings of _D. ferox_, immediately after germination,
had dark purple stems, extending from the young roots up to the
cotyledons, and that this tint remained ever afterwards as a ring round
the base of the stem of the plant when old. Now I have shown in the
thirteenth chapter that the retention or exaggeration of an early
character is so intimately related to reversion, that it evidently
comes under the same principle. Hence probably we ought to look at the
purple flowers and brown stems of these hybrids, not as new characters
due to variability, but as a return to the former state of some ancient
progenitor.

Independently of the appearance of new characters from crossing, a few
words may be added to what has been said in former chapters on the
unequal combination and transmission of the characters proper to the
two parent-forms. When two species or races are crossed, the offspring
of {267} the first generation are generally uniform, but subs