Part 21

Phil. Trans. 1900, vol. 195, A, p. 121.

But the truth is that even in 1898 such a comparison was scarcely happy. Not to mention moderns, these high hopes had been finally disposed of by the work of the experimental breeders such as Kölreuter, Knight, Herbert, Gärtner, Wichura, Godron, Naudin, and many more. To have treated as non-existent the work of this group of naturalists, who alone have attempted to solve the problems of heredity and species--Evolution, as we should now say--by the only sound method--experimental breeding--to leave out of consideration almost the whole block of evidence collected in Animals and Plants--Darwin’s finest legacy as I venture to declare--was unfortunate on the part of any exponent of Heredity, and in the writings of a professed naturalist would have been unpardonable. But even as modified in 1900 the Law of Ancestral Heredity is heavily over-sparred, and any experimental breeder could have increased Pearson’s list of unconformable cases by as many again.

But to return to Professor Weldon. He now repeats that the Law of Ancestral Heredity seems likely to prove generally applicable to blended inheritance, but that the case of alternative inheritance is for the present reserved. We should feel more confidence in Professor Weldon’s exposition if he had here reminded us that the special case which fitted Galton’s Law so well that it emboldened him to announce that principle as apparently “universally applicable to bi-sexual descent” was one of alternative inheritance--namely the coat-colour of Basset-hounds. Such a fact is, to say the least, ominous. Pearson, in speaking (1900) of this famous case of Galton’s, says that these phenomena of alternative inheritance must be treated separately (from those of blended inheritance), and for them he deduces a proposed “law of reversion,” based of course on ancestry. He writes, “In both cases we may speak of a law of ancestral heredity, but the first predicts the probable character of the individual produced by a given ancestry, while the second tells us the percentages of the total offspring which on the average revert to each ancestral type.”

“If this be done, we shall, I venture to think, keep not only our minds, but our points for observation, clearer; and further, the failure of Mr Galton’s statement in the one case will not in the least affect its validity in the other.” Pearson (32), p. 143.

Grammar of Science, 1900, p. 494. See also Pearson, Proc. Roy. Soc. 1900, LXVI. pp. 142–3.

With the distinctions between the original Law of Ancestral Heredity, the modified form of the same law, and the Law of Reversion, important as all these considerations are, we are not at present concerned.

For the Mendelian principle of heredity asserts a proposition absolutely at variance with all the laws of ancestral heredity, however formulated. In those cases to which it applies strictly, this principle declares that the cross-breeding of parents need not diminish the purity of their germ-cells or consequently the purity of their offspring. When in such cases individuals bearing opposite characters, A and B, are crossed, the germ-cells of the resulting cross-bred, AB, are each to be bearers either of character A or of character B, not both.

Consequently when the cross-breds breed either together or with the pure forms, individuals will result of the forms AA, AB, BA, BB. Of these the forms AA and BB, formed by the union of similar germs, are stated to be as pure as if they had had no cross in their pedigree, and henceforth their offspring will be no more likely to depart from the A type or the B type respectively, than those of any other originally pure specimens of these types.

On an average of cases, in equal numbers, as Mendel found.

Consequently in such examples it is not the fact that each ancestor must be brought to account as the Galton-Pearson Law asserts, and we are clearly dealing with a physiological phenomenon not contemplated by that Law at all.

Every case therefore which obeys the Mendelian principle is in direct contradiction to the proposition to which Professor Weldon’s school is committed, and it is natural that he should be disposed to consider the Mendelian principle as applying especially to “alternative” inheritance, while the law of Galton and Pearson is to include the phenomenon of blended inheritance. The latter, he tells us, is “the most usual case,” a view which, if supported by evidence, might not be without value.

It is difficult to blame those who on first acquaintance concluded Mendel’s principle can have no strict application save to alternative inheritance. Whatever blame there is in this I share with Professor Weldon and those whom he follows. Mendel’s own cases were almost all alternative; also the fact of dominance is very dazzling at first. But that was two years ago, and when one begins to see clearly again, it does not look so certain that the real essence of Mendel’s discovery, the purity of germ-cells in respect of certain characters, may not apply also to some phenomena of blended inheritance. The analysis of this possibility would take us to too great length, but I commend to those who are more familiar with statistical method, the consideration of this question: whether dominance being absent, indefinite, or suppressed, the phenomena of heritages completely blended in the zygote, may not be produced by gametes presenting Mendelian purity of characters. A brief discussion of this possibility is given in the Introduction, p. 31.

Very careful inquiry would be needed before such a possibility could be negatived. For example, we know that the Laws based on Ancestry can apply to alternative inheritance; witness the case of the Basset-hounds. Here there is no simple Mendelian dominance; but are we sure there is no purity of germ-cells? The new conception goes a long way and it may well reach to such facts as these.

But for the present we will assume that Mendel’s principle applies only to certain phenomena of alternative inheritance, which is as far as our warrant yet runs.

No close student of the recent history of evolutionary thought needs to be told what the attitude of Professor Weldon and his followers has been towards these same disquieting and unwelcome phenomena of alternative inheritance and discontinuity in variation. Holding at first each such fact for suspect, then treating them as rare and negligible occurrences, he and his followers have of late come slowly to accede to the facts of discontinuity a bare and grudging recognition in their scheme of evolution.

Read in this connexion Pearson, K., Grammar of Science, 2nd ed. 1900, pp. 390–2.

Professor Weldon even now opens his essay with the statement--or perhaps reminiscence--that “it is perfectly possible and indeed probable that the difference between these forms of inheritance [blended, mosaic, and alternative] is only one of degree.” This may be true; but reasoning favourable to this proposition could equally be used to prove the difference between mechanical mixture and chemical combination to be a difference of degree.

Therefore on the announcement of that discovery which once and for all ratifies and consolidates the conception of discontinuous variation, and goes far to define that of alternative inheritance, giving a finite body to what before was vague and tentative, it is small wonder if Professor Weldon is disposed to criticism rather than to cordiality.

* * * * *

We have now seen what is the essence of Mendel’s discovery based on a series of experiments of unequalled simplicity which Professor Weldon does not venture to dispute.

II. MENDEL AND THE CRITIC’S VERSION OF HIM.

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.”

III. THE FACTS IN REGARD TO DOMINANCE OF CHARACTERS IN PEAS.