Heimans (1970) bases his case that Mendel opposed the notion of ‘constant hybrids’ on a rough note (or Notizblatt) that has been found in Mendel's handwriting—which Heimans calculates was written around 1876 or 1877; it was scribbled on the back of some scrap paper.1 This is an extreme case, in fact, of an elaborate theory constructed around a slender textual fragment. Mendel's note includes the following:
Mendel notes: 'Bei Veränderlich Ausgleichung—hier nicht'.2 This is followed by a series of numbers:
In the 1876 note, Mendel follows the series of numbers with the words: 'doch nie so anwachCr, wie bei Salix'. Heimans assumes 'anwachCr' must mean 'anwachsende Combinationsreihen'—'cumulative series of combinations'. Combinationsreihe is used by Mendel in his Pisum paper to refer to the various combinations of character traits in hybrids, as observed over successive generations. The whole phrase in Mendel's note means: 'however [doch], never such cumulative series of combinations, as in Salix'. The lack of any other information or context means that Mendel's meaning here is unclear. The note could imply, 'you never get such cumulative Combinationsreihen as you get in Salix'; or the opposite: 'however, as in Salix, you never get such cumulative Combinationsreihen'.
Heimans goes for Option One, and elaborates on it. What Mendel was trying to say, he claims, was 'that in Salix, more than in any other progeny, fertile and normally segregating hybrids are formed between parent species differing in a great number of characters'. Thus, according to Heimans, Mendel had found that Salix hybrids were not constant at all, but segregated like pea plants; and in this way, Mendel's faith in his laws of variation and segregation was affirmed. (Hieracium plants, which did not conform to Pisum laws, were evidently put aside and 'for the time, abandoned'—p. 22.)
Hitherto, of course, Mendel had relied on Wichura's testimony that Salix hybrids are constant. There is actually no evidence that Mendel himself experimented with Salix hybrids at all—let alone that he found that they followed his Pisum laws more reliably than other species. Heiman's case is based on a questionable translation and interpretation of a single line.
If we take Mendel's note to mean—as in Option Two—'however, just as in Salix, you never get such cumulative Combinationsreihen', then the implication is very different. Neither Hieracium or Salix follow the usual pattern for ‘variable’ hybrids. This reading of Mendel's note is consistent with statements Mendel made elsewhere about Salix and Hieracium as constant hybrids. Moreover, Option Two is grammatically the more accurate translation. (Alan Jones has observed that the placing of a comma after ‘anwachCr’ is crucial: in German, this gives the line the meaning, ‘just as you don't get them in Salix’. Without the comma, Jones suggests, ‘it would be the other meaning’.3)
But however we interpret the note, it is too slight a fragment to build a whole case upon. Heimans, in fact, skews the evidence in order to serve his agenda: to show that Mendel was a lone fighter against the very idea of constant hybrids. Others have taken up Heiman's argument and presented it as fact. Thus, solely on the basis of Heiman's shaky evidence, Orel concluded that Mendel finally found even the 'apparently constant' Hieracium hybrid to be 'in agreement with the theory of variable hybrids' (1998, p. 299). (Even Heimans did not claim this.)
2. The word 'veränderlich' is an adjective. It is odd that Mendel gives the word a capital letter (which is grammatically incorrect), and does not decline it to match the noun ('Ausgleichung').
3. Alan Jones, formerly Principal Lecturer in German, University of Hertfordshire. Email communication, 7 December 2009.
Most shared on Macroevolution.net: