seeds is larger than in the first crop, and that the largest seeds are
now somewhat larger and more numerous. Again sowing these, we obtain a
further slight increase of size, and in a very few years we obtain a
greatly improved race, which will always produce larger seeds than the
unimproved race, even if cultivated without any special care. In this
way all our fine sorts of vegetables, fruits, and flowers have been
obtained, all our choice breeds of cattle or of poultry, our wonderful
race-horses, and our endless varieties of dogs. It is a very common but
mistaken idea that this improvement is due to crossing and feeding in
the case of animals, and to improved cultivation in the case of plants.
Crossing is occasionally used in order to obtain a combination of
qualities found in two distinct breeds, and also because it is found to
increase the constitutional vigour; but every breed possessing any
exceptional quality is the result of the selection of variations
occurring year after year and accumulated in the manner just described.
Purity of breed, with repeated selection of the best varieties of that
breed, is the foundation of all improvement in our domestic animals and
cultivated plants.

_Proofs of the Generality of Variation._

Another very common error is, that variation is the exception, and
rather a rare exception, and that it occurs only in one direction at a
time--that is, that only one or two of the numerous possible modes of
variation occur at the same time. The experience of breeders and
cultivators, however, proves that variation is the rule instead of the
exception, and that it occurs, more or less, in almost every direction.
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