A corolla cannot be at once monochromatic and striped, nor
can the same part of a stem be twisted and straight. But neighboring
organs may show the opposite attributes side by side.
In order to look closer into the real mechanism of this form of
variability, and of this constant tendency to occasional reversions, it
will be best to limit ourselves first to a single case, and to try to
gather all the evidence, which can be obtained by an examination of the
hereditary relations of its sundry constituents.
[314] This may best be done by determining the degree of inheritance for
the various constituents of the race during a series of years. It is
only necessary to apply the two precautions of excluding all
cross-fertilization, and of gathering the seeds of each individual
separately. We do not need to ascertain whether the variety as such is
permanent; this is already clear from the simple fact of its antiquity
in so many cases. We wish to learn what part each individual, or each
group of individuals with similar characters, play in the common line of
inheritance. In other words, we must build up a genealogical tree,
embracing several generations and a complete set of the single cases
occurring within the variety, in order to allow of its being considered
as a part of the genealogy of the whole.
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