When 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, I think we are
driven to conclude that this greater variability is simply 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 have been exposed under nature. There is, also, I
think, some probability in the view propounded by Andrew Knight, that
this variability may be partly connected with excess of food. It seems
pretty clear that organic beings must be exposed during several
generations to the new conditions of life to cause any appreciable
amount of variation; and that when the organisation has once begun to
vary, it generally continues to vary for many generations. No case is
on record of a variable being ceasing to be variable under
cultivation. Our oldest cultivated plants, such as wheat, still often
yield new varieties: our oldest domesticated animals are still capable
of rapid improvement or modification.
It has been disputed at what period of life the causes of variability,
whatever they may be, generally act; whether during the early or late
period of development of the embryo, or at the instant of conception.
Geoffroy St. Hilaire's experiments show that unnatural treatment of
the embryo causes monstrosities; and monstrosities cannot be separated
by any clear line of distinction from mere variations.
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