This is shown by the fact that different species of plants and animals
have required different _kinds_ of modification to adapt them to our
use, and we have never failed to meet with variation _in that particular
direction_, so as to enable us to accumulate it and so to produce
ultimately a large amount of change in the required direction. Our
gardens furnish us with numberless examples of this property of plants.
In the cabbage and lettuce we have found variation in the size and mode
of growth of the leaf, enabling us to produce by selection the almost
innumerable varieties, some with solid heads of foliage quite unlike any
plant in a state of nature, others with curiously wrinkled leaves like
the savoy, others of a deep purple colour used for pickling. From the
very same species 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.
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