The Early Opposition to Mendelism, 1900-1906

We all learn about Gregor Mendel’s pea experiments in school. The typical afterstory says that his papers from 1866 went unnoticed until Hugo de Vries, Carl Correns, and Erich von Tschermak-Seysenegg rediscovered them in 1900 and Mendel’s laws then quickly became mainstream biology.

As a summary, this is true, but it brushes over a big controversy that engulfed evolutionary circles between 1900 and 1906.

By 1900, that evolution occurs was not disputed anymore, but the Darwinian conception of natural selection and gradual evolution was taking a beating, and the environment was highly-charged and competitive. Seemingly out of nowhere, this grand new theory pops up, backed by many small experiments, and its influence spread swiftly and internationally.

This is not to say that Mendelism was unchallenged. The nature of the unit of heredity was the central problem to solve since Darwin’s work. On the one side, you had those scientists who postulated a material thing that was transmitted. Some examples:

  • Darwin called them gemmules;
  • Hugo de Vries called them pangenes;
  • Carl Nägeli called it the idioplasm;
  • August Weismann called it the germ plasm;

This group was not monolithic at all, encompassing Darwinists and neo-Lamarckians and saltationists, all of whom were at war with each other over the nature of evolution. One school of thought, led by William Bateson (pictured above), was especially interested in the role played by variations among individuals.

On the other side, you had a group of technical and mathematically-inclined scientists who viewed heredity not as a material substance, but as a force that accumulates over generations. These were the biometricians, and their preferred methods were statistics and positivism.

As soon as Mendelism came in the picture, the Batesonians flocked to it. The biometricians’ response was more varied. Some, like George Udny Yule (1902), tried to merge the viewpoints, but these attempts were unsuccessful, and the biometricians ended up being the Mendelians’ biggest opponents. The two people who spearheaded this opposition were Karl Pearson and Walter Frank Raphael Weldon.

by Elliott & Fry, albumen cabinet card, May 1890
by Elliott & Fry, albumen cabinet card, May 1890

Pearson was a Cambridge-educated mathematician, now mostly known for his Grammar of Science book, in which he developed his positivist and strictly empiricist philosophy of science (including sections on evolution in later editions). Read more about him in the masterful 2004 biography by Theodore Porter, Karl Pearson: The Scientific Life in a Statistical Age.

Weldon was a marine zoologist who became Professor of Zoology at University College, London in 1890, where he met Pearson, who was chair of Applied Mathematics and Mechanics there. Pearson introduced Weldon to statistics, Weldon introduced Pearson to biology, and from then on, they would collaborate intensely in applying statistics to biology, especially to evolution, with their research program focused on statistically measuring correlated variation in natural populations. Together, they founded the obviously-named journal Biometrika in 1901.

In the very first page of the first issue of the journal, Weldon outlined his view of how we should investigate evolution:

The starting point of Darwin’s theory of evolution is precisely the existence of those di fferences between individual members of a race or species which morphologists for the most part rightly neglect. The fi rst condition necessary, in order that a process of Natural Selection may begin among a race, or species, is the existence of diff erences among its members; and the first step in an enquiry into the possible eff ect of a selective process upon any character of a race must be an estimate of the frequency with which individuals, exhibiting any degree of abnormality with respect to that character, occur.

In other words, variation between individuals is not noise, it is what evolution and natural selection can act upon, and is exactly what should be studied. This was contra to the prevailing view pushed by Bateson, who dismissed the role of small variations and instead focused on larger, discontinuous variation (in modern terms, you would say he was more obsessed with disruptive selection).

By the time of the journal’s founding, battle lines had long been drawn already, from even before the appearance of Mendelism. Bateson and his colleagues were experimental scientists, while the biometricians were naturalists. One emphasised experiments, the other emphasised observations of nature (e.g. Weldon looked at crabs and snails), and this was where the split occurred. Note that these are modern labels, not what they referred to each other.

To us who have grown in the post-Modern Synthesis world, the reason behind this split seems really stupid, so please cast your modern knowledge aside for a moment. Back then, there were no clear concepts of “genotype” and “phenotype”. Those terms were coined by Danish botanist Wilhelm Johansen in 1892, before Mendel’s reintroduction to the world, and what they meant was unclear to everybody.

Phenotypes were seen as a source of continuous variation, individual differences between conspecifics (for example a gradient of roundness in peas). Genotypes were seen as sources of discontinuous variation, stark differences between conspecifics (for example, round vs wrinkled peas).

The biometricians supported Darwinian evolution by natural selection on continuous variation, while the Batesonians, now merged with the Mendelians, supported a saltationist view of evolution through new mutations. The biometricians could observe that variation in nature, in organismal phenotypes, and measure it, while the Mendelians could only find out genotype differences through experimentation, to the point of disregarding natural populations.

Importantly, this was a philosophical distinction that predated the rediscovery of Mendel’s laws. Mendelism merely poured fuel over an already-raging fire by finally providing concrete evidence of some sort of hereditary particle.

Weldon died prematurely of pneumonia in 1906, and with his death Pearson stopped working explicitly on evolution. This marked the victory of Bateson over the biometrician hold-offs, and with it, the triumph of Mendelism over the other evolutionary theories of the time.

This entire story is a demonstration of the power individual research groups have over the development of science. At the smallest scale, consider the strength of the Pearson-Weldon partnership that was so productive until Weldon’s early death prompted Pearson to lay off evolutionary studies, even though some members of his research group stayed on that track. At a medium scale, consider that the whole conflict began because of Weldon giving a bad review to Bateson’s book, Materials for the Study of Variation, in 1894. At the largest scale, consider that the Modern Synthesis of the 1940s was mostly about reconciling the philosophies expounded by the biometricians and the Mendelians, to state that they were both correct to some extent. This could have happened much earlier had the two sides bothered to listen to each other. That both Weldon and Bateson were students of Francis Galton, who instilled in both of them the same sense of the importance of variation in evolution, should have been a big hint.

Instead, through series of attacks, the wedge was driven deeper and deeper until each side thought the other was crazy, until it devolved to a state in which Bateson was claiming that Mendelian inheritance proves evolution by discontinuous mutations and that shows natural selection to be worthless, and Weldon claimed discontinuous variation didn’t follow Mendel’s laws. Had either Bateson or Weldon bothered to go out of their comfort zones and discuss the issues with other scientists, none of this would have happened, because all the silly extreme claims could have been shown to be wrong even with the science of the time. Instead, everybody lost.

With the Mendelist win over the biometricians (or, of Bateson over Weldon by way of premature death), discussion of continuous variation was more or less unheard of until geneticist Kenneth Majer and his lab’s work in the 1940s. The “win” was actually unjustified, considering that if you look at the issue using our modern understanding of genetics and evolution, Weldon was in some ways closer to the mark than Bateson. At least Weldon understood that variation was on a spectrum, one extreme of which was the discontinuous mutation stressed by Bateson. Thomas Hunt Morgan, three decades later in the Modern Synthesis, united the two views by saying that continuous variation was caused by mutations. Those are three decades wasted, and the reason for the delay comes down in no small part to personal animosity between Bateson and Weldon-Pearson. Let’s not forget that the reason the conflict started was because Bateson’s book was heavily criticised by Weldon and that made Bateson sad.

Documenting all the disagreements, chronologically or otherwise, is too large a task for a quick article, so let’s just look at one exchange. In a 1902 Biometrika article, Weldon claimed that when conducting genetic experiments, we should consider the entire ancestry of the parents when trying to predict what offspring will look like using Mendelian Laws. In other words, the F1 generation is not indicative of the whole species’s variability. A reasonable argument, to which Bateson issued an almost immediate response that listed a lot of data supporting the validiy of Mendelism, but also betrayed more personal opinions; particularly telling is Bateson’s worry that

coming from Professor Weldon there was the danger – almost the certainty – that the small band of younger men who are thinking of research in this field would take it they had learnt the gist of Mendel, would imagine his teaching exposed by Professor Weldon, and look elsewhere for lines of work.

In other words, he was afraid that Weldon’s criticisms were too effective and authoritarian and would result in young student turning away from the research program he prefers. In any case, the response was good, even if the Preface had more ad hominems than necessary. Weldon’s response in turn addressed Bateson’s case studies, showing how Darwinian selection could also give the same results as those Bateson ascribes to Mendelian inheritance.

The point from this example is that even back then, the fusion that will happen in a few decades was already apparent if the two sides were willing to stop trying to prove themselves right and the others wrong, and instead attempted to look at all the examples more objectively, especially since both Bateson and Weldon’s research groups had a lot of data they could call upon.

After Weldon’s death, the controversy waned and Mendelism became mainstream and commonly accepted, with Bateson and his collaborator Edith Saunders becoming the de-facto leaders in the research (and coining a lot of the genetics lexicon we still use, including the word “genetics”). The biometric view in evolution did persist in Pearson’s lab, despite Pearson himself not being active in it anymore. Interestingly, Pearson’s political, socialist, and feminist leanings led to his lab being comprised of around one-third women, which surely was a landmark for the time. They conducted research on the application of statistics to biology, meteorology, and sociology, and occasionally dipping back into heredity and evolution.

In the end, the main outcome of the biometricist-geneticist controversy was not the development of modern evolutionary theory, it was the development of theoretical population genetics, which was then used as one of the bases for the Modern Synthesis. Decades later, Bateson referred to this early period of genetics as “the utter darkness before the Mendelian dawn”.

As for Mendelism, it became the crown jewel of evolution, and even though genetics has grown a lot since its humble birth in a priest’s garden and its overly-hormonal puberty in Britain, Mendelism can still be seen as a clear underpinning rather than a distant, mostly-wrong, ancestor. Besides the fact that it’s a correct theory, social and political factors played a huge role in its very rapid acceptance. Knowing that the best way to gain credibility is to bring real-world applications, geneticists formed alliances with the agricultural sector, especially plant breeders. Both sides benefited a lot from this cooperation: the breeders could strengthen their craft with the power of genetically-guided breeding, resulting in new and better varieties, and the geneticists found out first-hand what exciting and crucial problems and research questions needed to be addressed – the more they could solve, the more mainstream acceptance they could earn. Through agriculture’s increased efficacy, genetics also earned political points, with varieties bred under the guidance of genetics to be more resistant to disease becoming favoured by law, forcing farmers to adopt them.

Further Reading:

A Cultural History of Heredity

by Staffan Müller-Wille [University Of Chicago Press, Hardcover]
Price: $43.32 £21.67 EUR 34,70 EUR 42,27 EUR 41,04 EUR 46,07 CDN$ 55.01


Bateson W. 1902. Mendel’s Principles of Heredity: A Defence.

Weldon WFR. 1902. Mendel’s Laws of Alternative Inheritance in Peas. Biometrika 1, 228-254.

Weldon WFR. 1902. On the Ambiguity of Mendel’s Categories. Biometrika 2, 44-55.

Yule GY. 1902. Mendel’s Laws and their Probably Relations to Intra-Racial Heredity. New Phytologist 1, 222-238.

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