“The experiments also tend to show that the height of the plant
is singularly influenced by crossing; a cross between two dwarf
peas, commonly producing some dwarf and some tall [? in the second
generation]; but on the other hand, a cross between two tall peas
does not exhibit a tendency to diminution in height.
“No perceptible difference appears to result from reversing the
parents; the influence of the pollen of each parent at the climax or
fourth generation producing similar results[142].”
[142] Laxton constantly refers to this conception of the “climax”
of--as we now perceive--analytical variation and recombination. Many
citations could be given respecting his views on this “climax” (cp.
p. 167).
The significance of this latter testimony I will presently discuss.
Professor Weldon next appeals to a later paper of Laxton’s published in
1890. From it he quotes this passage:
“By means, however, of cross-fertilisation alone, and unless it be
followed by careful and continuous selection, the labours of the
cross-breeder, instead of benefiting the gardener, may lead to utter
confusion,”
Here again the reader would have gained had Professor Weldon, instead
of leaving off at the comma, gone on to the end of the paragraph, which
proceeds thus:--
“because, as I have previously stated, the Pea under ordinary
conditions is much given to sporting and reversion, for when two
dissimilar old or fixed varieties have been cross-fertilised,
three or four generations at least must, under the most favourable
circumstances, elapse before the progeny will become fixed or
settled; and from one such cross I have no doubt that, by sowing
every individual Pea produced during the three or four generations,
hundreds of different varieties may be obtained; but as might be
expected, I have found that where the two varieties desired to be
intercrossed are unfixed, confusion will become confounded[143],
and the variations continue through many generations, the number at
length being utterly incalculable.”
[143] Further subdivision and recombination of hypallelomorphs.
Professor Weldon declares that Laxton’s “experience was altogether
different from that of Mendel.” The reader will bear in mind that when
Laxton speaks of fixing a variety he is not thinking particularly of
seed-characters, but of all the complex characters, fertility, size,
flavour, season of maturity, hardiness, etc., which go to make a
serviceable pea. Considered carefully, Laxton’s testimony is so closely
in accord with Mendelian expectation that I can imagine no chance
description in non-Mendelian language more accurately stating the
phenomena.
Here we are told in unmistakable terms the breaking up of the original
combination of characters on crossing, their re-arrangement, that
at the fourth or fifth generation the possibilities of sporting
[sub-division of compound allelomorphs and re-combinations of them?]
are exhausted, that there are then definite forms which if selected
are thenceforth fixed [produced by union of similar gametes?]
that it takes longer to select some forms [dominants?] than others
[recessives?], that there may be “mule” forms[144] or forms which
cannot be fixed at all[145] [produced by union of dissimilar gametes?].
[144] For instance the _talls_ produced by crossing _dwarfs_ are such
“mules.” Tschermak found in certain cases distinct increase in height
in such a case, though not always (p. 531).
[145] “The remarkably fine but unfixable pea _Evolution_.” Laxton, p.
37.
But Laxton tells us more than this. He shows us that numbers of
varieties may be obtained--hundreds--“incalculable numbers.” Here
too if Professor Weldon had followed Mendel with even moderate care
he would have found the secret. For in dealing with the crosses of
_Phaseolus_ Mendel clearly forecasts the conception of _compound
characters themselves again consisting of definite units_, all of which
may be separated and re-combined in the possible combinations, laying
for us the foundation of the new science of Analytical Biology.
How did Professor Weldon, after reading Mendel, fail to perceive these
principles permeating Laxton’s facts? Laxton must have seen the very
things that Mendel saw, and had he with his other gifts combined that
penetration which detects a great principle hidden in the thin mist of
“exceptions,” we should have been able to claim for him that honour
which must ever be Mendel’s in the history of discovery.
When Laxton speaks of selection and the need for it, he means, what
the raiser of new varieties almost always means, the selection of
_definite_ forms, not impalpable fluctuations. When he says that
without selection there will be utter confusion, he means--to use
Mendelian terms--that the plant which shows the desired combination of
characters must be chosen and bred from, and that if this be not done
the grower will have endless combinations mixed together in his stock.
If however such a selection be made in the fourth or fifth generation
the breeder may very possibly have got a fixed form--namely, one that
will breed true[146]. On the other hand he may light on one that does
not breed true, and in the latter case it may be that the particular
type he has chosen is not represented in the gametes and will _never_
breed true, though selected to the end of time. Of all this Mendel has
given us the simple and final account.
[146] Apart from fresh original variations, and perhaps in some cases
imperfect homozygosis of some hypallelomorphs.
At Messrs Sutton and Sons, to whom I am most grateful for unlimited
opportunities of study, I have seen exactly such a case as this. For
many years Messrs Sutton have been engaged in developing new strains
of the Chinese Primrose (_Primula sinensis_, hort.). Some thirty
thoroughly distinct and striking varieties (not counting the _Stellata_
or “Star” section) have already been produced which breed true or
very nearly so. In 1899 Messrs Sutton called my attention to a strain
known as “Giant Lavender,” a particularly fine form with pale magenta
or lavender flowers, telling me that it had never become fixed. On
examination it appeared that self-fertilised seed saved from this
variety gave some magenta-reds, some lavenders, and some which are
white on opening but tinge with very faint pink as the flower matures.
On counting these three forms in two successive years the following
figures appeared. Two separately bred batches raised from “Giant
Lavender” were counted in each year.
Magenta Lavender White
red faintly tinged
1901 1st batch 19 27 14
" 2nd " 9 20 9
1902 1st " 12 23 11
" 2nd " 14 26 11
-- -- --
54 96 45
The numbers 54 : 96 : 45 approach the ratio 1 : 2 : 1 so nearly that
there can be no doubt we have here a simple case of Mendelian laws,
operating without definite dominance, but rather with blending.
When Laxton speaks of the “remarkably fine but unfixable pea
_Evolution_” we now know for the first time exactly what the phenomenon
meant. It, like the “Giant Lavender,” was a “mule” form, not
represented by germ-cells, and in each year arose by “self-crossing.”
This is only one case among many similar ones seen in the Chinese
Primrose. In others there is no doubt that more complex factors are at
work, the subdivision of compound characters, and so on. The history
of the “Giant Lavender” goes back many years and is not known with
sufficient precision for our purposes, but like all these forms it
originated from crossings among the old simple colour varieties of
_sinensis_.