Such experiments however, have miscarried owing to the too stable nature
of the unit-characters.
This stability and this absence of the splitting shown by varietal marks
in the offspring of hybrids is one of the best proofs of unisexual
unions. It is often obscured by the accompanying varietal marks, or
overlooked for this reason. Only in rare cases it is to be met with in a
pure state and some examples are given of this below.
Before doing so, I must call your attention to another feature of the
unbalanced unions. This is the diminution of the fertility, a phenomenon
universally known as occurring in hybridizations. It has two phases.
First, the diminished chance of the crosses themselves of giving full
crops of seed, as compared with the pure fertilization of either parent.
And, secondly, the fertility of the hybrids themselves. Seemingly, all
grades of diminished fertility occur and the oldest authors on hybrids
have pointed out that a very definite relation exists between the
differences of the parents and the degree of sterility, both of the
cross and of the hybrid offspring. In a broad sense these two factors
are proportionate to each other, the sterility being the greater, the
lesser the affinity between the parents. Many writers have [262] tried
to trace this rule in the single cases, but have met with nearly
unsurmountable difficulties, owing chiefly to our ignorance of the units
which form the differences between the parents in the observed cases.
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