as the cabbage (Brassica oleracea) have arisen the
broccoli and cauliflower, in which the leaves have undergone little
alteration, while the branching heads of flowers grow into a compact
mass forming one of our most delicate vegetables. The brussels sprouts
are another form of the same plant, in which the whole mode of growth
has been altered, numerous little heads of leaves being produced on the
stem. In other varieties the ribs of the leaves are thickened so as to
become themselves a culinary vegetable; while, in the Kohlrabi, the stem
grows into a turnip-like mass just above ground. Now all these
extraordinarily distinct plants come from one original species which
still grows wild on our coasts; and it must have varied in all these
directions, otherwise variations could not have been accumulated to the
extent we now see them. The flowers and seeds of all these plants have
remained nearly stationary, because no attempt has been made to
accumulate the slight variations that no doubt occur in them.

If now we turn to another set of plants, the turnips, radishes, carrots,
and potatoes, we find that the roots or underground tubers have been
wonderfully enlarged and improved, and also altered in shape and colour,
while the stems, leaves, flowers, and fruits have remained almost
unchanged. In the various kinds of peas and beans it is the pod or fruit
and the seed that has been subjected to selection, and therefore greatly
modified; and it is here very important to notice that while all these
plants have undergone cultivation in a great variety of soils and
climates, with different manures and under different systems, yet the
flowers have remained but little altered, those of the broad bean, the
scarlet-runner, and the garden-pea, being nearly the same in all the
varieties. This shows us how little change is produced by mere
cultivation, or even by variety of soil and climate, if there is no
_selection_ to preserve and accumulate the small variations that are
continually occurring. When, however, a great amount of modification has
been effected in one country, change to another country produces a
decided effect. Thus it has been found that some of the numerous
varieties of maize produced and cultivated in the United States change
considerably, not only in their size and colour, but even in the shape
of the seed when grown for a few successive years in Germany.[31] In all
our cultivated fruit trees the fruits vary immensely