t apart. In fact, this is the way the
cultivated domesticated races, so useful to man, have been fixed and
preserved. Races, in fact, can hardly, if at all, be said to exist
independently of man. But man does not really produce them. Such
peculiarities--often surprising enough--now and then originate, we know
not how (the plant _sports_, as the gardeners say); they are only
preserved, propagated, and generally further developed, by the
cultivator's skilful care. If left alone, they are likely to dwindle and
perish, or else revert to the original form of the species. Vegetable
races are commonly annuals, which can be kept up only by seed, or herbs
of which a succession of generations can be had every year or two, and
so the education by selection be completed without great lapse of time.
But all fruit-trees could probably be fixed into races in an equal
number of generations.

BUD-VARIETIES are those which spring from buds instead of seed. They are
uncommon to any marked extent. They are sometimes called _Sports_, but
this name is equally applied to variations among seedlings.

CROSS-BREEDS, strictly so-called, are the variations which come from
cross-fertilizing one variety of a species with another.

HYBRIDS are the varieties, if they may be so called,--which come from
the crossing of species (331). Only nearly related species can be
hybridized; and the resulting progeny is usually self-sterile, but not
always. Hybrid plants, however, may often be fertilized and made
prolific by the pollen of one or the other parent. This produces another
kind of cross-breeds.

525. Species are the units in classification. Varieties, although of
utmost importance in cultivation and of considerable consequence in the
flora of any country, are of less botanical significance. For they are
apt to be indefinite and to shade off one form into another. But
species, the botanist _expects_ to be distinct. Indeed, the practical
difference to the botanist between species and varieties is the definite
limitation of the one and the indefiniteness of the other. The
botanist's determination is partly a matter of observation, partly of
judgment.

526. In an enlarged view, varieties may be incipient species; and nearly
related species probably came from a common stock in earlier times. For
there is every reason to believe that existing vegetation came from the
more or less changed vegetation of a preceding geological era. However
that may be, spe