They require pairs of
antagonistic qualities, and have no connection whatever with those
qualities, which do not find an opponent in the other parent. Now, only
pure varieties afford such pure conditions. When undergoing further
modifications, some of them may be in the progressive line and others in
the retrogressive. Progressive modifications give new units, which are
not in contrast with any other, retrograde changes turn active units
into the latent condition and so give rise to pairs. Ordinary species
generally originate in this way, and hence differ from each other partly
in specific, partly in varietal characters. As to the first, they give
in their hybrids stable peculiarities, while as to the latter, they
split up according to Mendel's law.
Unpaired or unbalanced characters lie side by side with paired or
balanced qualities, and they [308] do so in nearly all the crosses made
for practical purposes, and in very many scientific experiments. Even
Mendel's peas were not pure in this respect, much less do the campions
noted above differ only in Mendelian characters.
Comparative and systematic studies must be made to ascertain the true
nature of every unit in every single plant, and crossing experiments
must be based on these distinctions in order to decide what laws are
applicable in any case.
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