Domestic Varieties from one or more Species -- Domestic Pigeons, their
Differences and Origin -- Principles of Selection, anciently followed,
their Effects -- Methodical and Unconscious Selection -- Unknown Origin of
our Domestic Productions -- Circumstances favourable to Man's power of
Selection.

CAUSES OF VARIABILITY.

When we compare the individuals of the same variety or sub-variety of our
older cultivated plants and animals, one of the first points which strikes
us is, that they generally differ more from each other than do the
individuals of any one species or variety in a state of nature. And if we
reflect on the vast diversity of the plants and animals which have been
cultivated, and which have varied during all ages under the most different
climates and treatment, we are driven to conclude that this great
variability is due to our domestic productions having been raised under
conditions of life not so uniform as, and somewhat different from, those to
which the parent species had been exposed under nature. There is, also,
some probability in the view propounded by Andrew Knight, that this
variability may be partly connected with excess of food. It seems clear
that organic beings must be exposed during several generations to new
conditions to cause any great amount of variation; and that, when the
organisation has once begun to vary, it generally continues varying for
many generations. No case is on record of a variable organism ceasing to
vary under cultivation. Our oldest cultivated plants, such as wheat, still
yield new varieties: our oldest domesticated animals are still capable of
rapid improvement or modification.

As far as I am able to judge, after long attending to the subject, the
conditions of life appear to act in two ways--directly on the whole