multiplied by buds and cuttings, hybridisation has done wonders, as with many
kinds of Roses, Rhododendrons, Pelargoniums, Calceolarias, and Petunias.
Nearly all these plants can be propagated by seed, most of them freely; but
extremely few or none come true by seed.

Some authors believe that crossing is the chief cause of variability,--that
is, of the appearance of absolutely new characters. Some have gone so far as
to look at it as the sole cause; but this conclusion is disproved by the facts
given in the chapter on Bud-variation. The belief that characters not present
in either parent or in their ancestors frequently originate from crossing is
doubtful; that they occasionally do so is probable; but this subject will be
more conveniently discussed in a future chapter on the causes of Variability.

A condensed summary of this and of the three following chapters, together with
some remarks on Hybridism, will be given in the nineteenth chapter.


CHAPTER 2.XVI.

CAUSES WHICH INTERFERE WITH THE FREE CROSSING OF VARIETIES--INFLUENCE OF
DOMESTICATION ON FERTILITY.

DIFFICULTIES IN JUDGING OF THE FERTILITY OF VARIETIES WHEN CROSSED.
VARIOUS CAUSES WHICH KEEP VARIETIES DISTINCT, AS THE PERIOD OF BREEDING AND
SEXUAL PREFERENCE.
VARIETIES OF WHEAT SAID TO BE STERILE WHEN CROSSED.
VARIETIES OF MAIZE, VERBASCUM, HOLLYHOCK, GOURDS, MELONS, AND TOBACCO,
RENDERED IN SOME DEGREE MUTUALLY STERILE.
DOMESTICATION ELIMINATES THE TENDENCY TO STERILITY NATURAL TO SPECIES WHEN
CROSSED.