Until now we have limited ourselves to the consideration of single
differentiating units. This discussion gives a clear insight into the
fundamental phenomena of hybrid fertilization. It at once shows the
correctness of the assumption of unit-characters, and of their pairing
in the sexual combinations.
But Mendel's law is not at all restricted to these simple cases. Quite
on the contrary, it explains the most intricate questions of
hybridization, providing they do not transgress the limits of
symmetrical unions. But in this realm nearly all results may be
calculated beforehand, on the ground of the principle of probability.
Only one more assumption need be discussed. The several pairs of
antagonistic characters must be independent from, and uninfluenced by,
one another. This premise seems to hold good in the vast majority of
cases, though rare exceptions seem to be not entirely wanting. Hence the
necessity of taking all predictions from Mendel's law only as
probabilities, which will prove true in most, but not necessarily in all
cases. [300] But here we will limit ourselves to normal cases.
The first example to be considered is obviously the assumption that the
parents of a cross differ from each other in respect to two characters.
A good illustrative example is afforded by the thorn-apple.
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