No naturalist who had read Galton’s paper and had tried to apply it to the facts he knew could fail to see that here was a definite advance. We could all perceive phenomena that were in accord with it and there was no reasonable doubt that closer study would prove that accord to be close. It was indeed an occasion for enthusiasm, though no one acquainted with the facts of experimental breeding could consider the suggestion of universal application for an instant.
But two years have gone by, and in 1900 Pearson writes[57] that the values obtained from the Law of Ancestral Heredity
“seem to fit the observed facts fairly well in the case of blended inheritance. In other words we have a certain amount of evidence in favour of the conclusion: That whenever the sexes are equipotent, blend their characters and mate pangamously, all characters will be inherited at the same rate,”
or, again in other words, that the Law of Ancestral Heredity after the glorious launch in 1898 has been home for a complete refit. The top-hamper is cut down and the vessel altogether more manageable; indeed she looks trimmed for most weathers. Each of the qualifications now introduced wards off whole classes of dangers. Later on (pp. 487–8) Pearson recites a further list of cases regarded as exceptional. “All characters will be inherited at the same rate” might indeed almost be taken to cover the results in Mendelian cases, though the mode by which those results are arrived at is of course wholly different.
Clearly we cannot speak of the Law of Gravitation now. Our Tycho Brahe and our Kepler, with the yet more distant Newton, are appropriately named as yet to come[58].
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)[59], 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[60].”
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[61]. 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.