manner as we have often seen with domesticated
animals, the supposed multiple origin of the cabbage throws no light on
the characteristic differences between the cultivated forms. If our
cabbages are the descendants of three or four distinct species, every
trace of any sterility which may originally have existed between them
is now lost, for none of the varieties can be kept distinct without
scrupulous care to prevent intercrossing.

The other cultivated forms of the genus Brassica are descended,
according to the view adopted by Godron and Metzger,[590] from two
species, _B. napus_ and _rapa_; but according to other botanists from
three species; whilst others again strongly suspect that all these
forms, both wild and cultivated, ought to be ranked as a single
species. _Brassica napus_ has given rise to two large groups, namely,
Swedish turnips (by some believed to be of hybrid origin)[591] and
Colzas, the seeds of which yield oil. _Brassica rapa_ (of Koch) has
also given rise to two races, namely, common turnips and the oil-giving
rape. The evidence is unusually clear that these latter plants, though
so different in external appearance, belong to the same species; for
the turnip has been observed by Koch and Godron to lose its thick roots
in uncultivated soil, and when rape and turnips are sown together they
cross to such a degree that scarcely a single plant comes true.[592]
Metzger by culture converted the biennial or winter rape into the
annual or summer rape,--varieties which have been thought by some
authors to be specifically distinct.[593]

In the production of large, fleshy, turnip-like stems, we have a case
{326} of analogous variation in three forms which are generally
considered as distinct species. But scarcely any modification seems so
easily acquired as a succulent enlargement of the stem or root--that is
a store of nutriment laid up for the plant's own future use. We see
this in our radishes, beet, and in the less generally known
"turnip-rooted" celery, and in the finocchio or Italian variety of the
common fennel. Mr. Buckman has lately proved by his interesting
experiments how quickly the roots of the wild parsnip can be enlarged,
as Vilmorin formerly proved in the case of the carrot.[594] This latter
plant, in its cultivated state, differs in scarcely any character from