_The “Law of Dominance.”_
I proceed to the question of dominance which Professor Weldon treats as
a prime issue, almost to the virtual concealment of the great fact of
gametic purity.
Cross-breds in general, _AB_ and _BA_, named above, may present many
appearances. They may all be indistinguishable from _A_, or from _B_;
some may appear _A_’s and some _B_’s; they may be patchworks of both;
they may be blends presenting one or many grades between the two; and
lastly they _may have an appearance special to themselves_ (_being in
the latter case, as it often happens, “reversionary”_), a possibility
which Professor Weldon does not stop to consider, though it is the clue
that may unravel many of the facts which mystify him now.
Mendel’s discovery became possible because he worked with regular
cases of the first category, in which he was able to recognize that
_one_ of each of the pairs of characters he studied _did_ thus prevail
and _was_ “dominant” in the cross-bred to the exclusion of the other
character. This fact, which is still an accident of particular cases,
Professor Weldon, following some of Mendel’s interpreters, dignifies by
the name of the “Law of Dominance,” though he omits to warn his reader
that Mendel states no “Law of Dominance” whatever. The whole question
whether one or other character of the antagonistic pair is dominant
though of great importance is logically a subordinate one. It depends
on the specific nature of the varieties and individuals used, sometimes
probably on the influence of external conditions and on other factors
we cannot now discuss. There is as yet no universal law here perceived
or declared.
Professor Weldon passes over the proof of the purity of the germ-cells
lightly enough, but this proposition of dominance, suspecting its
weakness, he puts prominently forward. Briefest equipment will
suffice. Facing, as he supposes, some new pretender--some local
Theudas--offering the last crazy prophecy,--any argument will do
for such an one. An eager gathering in an unfamiliar literature, a
scrutiny of samples, and he will prove to us with small difficulty that
dominance of yellow over green, and round over wrinkled, is irregular
even in peas after all; that in the sharpness of the discontinuity
exhibited by the variations of peas there are many grades; that many
of these grades co-exist in the same variety; that some varieties may
perhaps be normally intermediate. All these propositions are supported
by the production of a collection of evidence, the quality of which we
shall hereafter consider. “Enough has been said,” he writes (p. 240),
“to show the grave discrepancy between the evidence afforded by
Mendel’s own experiments and that obtained by other observers, equally
competent and trustworthy.”
We are asked to believe that Professor Weldon has thus discovered “a
fundamental mistake” vitiating all that work, the importance of which,
he elsewhere tells us, he has “no wish to belittle.”