Crosses of different varieties of the same species with
one another obviously constitute a derivative case, and should only be
discussed secondarily. And crosses of varieties with positive or
depressive characters have as yet so rarely been made that we may well
disregard them.
Elementary species differ from their nearest allies by progressive
changes, that is by the acquirement [253] of some new character. The
derivative species has one unit more than the parent. All other
qualities are the same as in the parent. Whenever such a derivative is
combined with its parent the result for these qualities will be exactly
as in a normal fertilization. In such ordinary cases it is obvious that
each character of the pollen-parent is combined with the same character
of the pistil-parent. There may be slight individual differences, but
each unit character will become opposed to, and united with, the same
unit-character in the other parent. In the offspring the units will thus
be paired, each pair consisting of two equivalent units. As to their
character the units of each single pair are the same, only they may
exhibit slight differences as to the degree of development of this
character.
Now we may apply this conception to the sexual combination of two
different elementary species, assuming one to be the derivative of the
other.
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