Even the rare pale
yellowish species of some of these genera have no tendency in this
direction. The hyacinths are the most remarkable, if not the sole known
instance of a species having red and blue and white and yellow
varieties, but here the yellow is not the bright golden color of the
buttercups.
The existence of varietal colors in allied species obviously points to a
common cause, and this cause can be no other than the latency of the
pigment in the species that do not show it.
The conception of latency of characters as the common source of the
origination of varieties, either in the positive or in the negative way,
leads to some rules on variability, which are known under the names
given to them by Darwin. They are the rules of repeated, homologous,
parallel and analogous variability. Each of them is quite general, and
may be recognized in instances from the most widely distant families.
Each of them is quite evident and easily understood on the principle of
latency.
By the term of repeated variability is meant the well-known phenomenon,
that the same variety has sprung at different times and in different
[243] countries from the same species. The repetition obviously
indicates a common internal cause. The white varieties of blue- and
red-flowered plants occur in the wild state so often, and in most of the
instances in so few individuals that a common pedigree is absolutely
improbable.
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