It may be doubted whether many permanent varieties have been formed by intentional or accidental crosses; for such crossed plants are found to be very inconstant. One kind, however, called "Cottager's Kail," has lately been produced by crossing common kail and Brussel- sprouts, recrossed with purple broccoli (9/71. 'Gardener's Chronicle' February 1858 page 128.), and is said to be true; but plants raised by me were not nearly so constant in character as any common kind of cabbage.
Although most of the kinds keep true if carefully preserved from crossing, yet the seed-beds must be yearly examined, and a few seedlings are generally found false; but even in this case the force of inheritance is shown, for, as Metzger has remarked (9/72. 'Kohlarten' s. 22.) when speaking of Brussel-sprouts, the variations generally keep to their "unter art," or main race. But in order that any kind may be truly propagated there must be no great change in the conditions of life; thus cabbages will not form heads in hot countries, and the same thing has been observed with an English variety grown during an extremely warm and damp autumn near Paris. (9/73. Godron 'De l'Espece' tome 2 page 52; Metzger 'Kohlarten' s. 22.) Extremely poor soil also affects the characters of certain varieties.
Most authors believe that all the races are descended from the wild cabbage found on the western shores of Europe; but Alph. De Candolle (9/74. 'Geograph. Bot.' page 840.) forcibly argues, on historical and other grounds, that it is more probable that two or three closely allied forms, generally ranked as distinct species, still living in the Mediterranean region, are the parents, now all commingled together, of the various cultivated kinds. In the same 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 (9/75. Godron 'De l'Espece' tome 2 page 54; Metzger 'Kohlarten' s. 10.), 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 (believed to be of hybrid origin) (9/76. 'Gardener's Chronicle and Agricult. Gazette' 1856 page 729. See more especially ibid 1868 page 275: the writer asserts that he planted a variety of cabbage (B. oleracea) close to turnips (B. rapa) and raised from the crossed seedlings true Swedish turnips. These latter plants ought, therefore, to be classed with cabbages or turnips, and not under B. napus.) 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. (9/77. 'Gardener's Chronicle and Agricult. Gazette' 1855 page 730.) 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. (9/78. Metzger, 'Kohlarten' s. 51.)
In the production of large, fleshy, turnip-like stems, we have a case 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.