The term
balanced crosses then conveys the idea of complete bisexuality, all
unit-characters combining in pairs. Unbalanced crosses are those in
which one or more units do not find their mates and therefore remain
unpaired. This distinction was proposed by Macfarlane when studying the
minute structure of plant-hybrids in comparison with that of their
parents (1892).
[256] In the first place it shows that a species hybrid may inherit the
distinguishing marks of both parents. In this way it may become
intermediate between them, having some characters in common with the
pollen-parent and others with the pistil-parent. As far as these
characters do not interfere with each other, they may be fully developed
side by side, and in the main this is the way in which hybrid characters
are evolved. But in most cases our existing knowledge of the units is
far too slender to give a complete analysis, even of these
distinguishing marks alone. We recognize the parental marks more or less
clearly, but are not prepared for exact delimitations. Leaving these
theoretical considerations, we will pass to the description of some
illustrative examples.
In the first place I will describe a hybrid between two species of
_Oenothera_, which I made some years ago. The parents were the common
evening-primrose or _Oenothera biennis_ and of its small-flowered
congener, _Oenothera muricata_.
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