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[62].
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.”