Mendel's work should have given evolutionary theory a boost, revealing the mechanism for Darwin's variation, but instead it bogged things down. Now the theory faced a morass of contention made deeper by hard feelings. Two factions, one led by William Bateson and the other by Walter Weldon and his teacher, Karl Pearson, used Mendel's findings against each other, and their arguments quickly got personal. Their prime area of disagreement was over whether evolution was smooth and gradual or jumpy and discontinuous. They also disagreed in a fundamental way about each other's scientific methods.
Weldon investigated the evolution of crabs in Plymouth Sound, a highly polluted waterway. He noticed that crabs there had wider carapaces than the same species in other waterways, and he guessed this had to do with having a greater gill surface, making them better able to filter out pollutants. These crabs would be more fit, better able to survive and reproduce, and he set out to prove this in the lab. He and his wife collected thousands of crabs, raising them in five hundred large bottles filled with the sewage-rich waters of the Sound. He described the work as "horrible from the great quantity of decaying matter necessary to kill a healthy crab."
The smells may have been bad, but the numbers were good. Weldon was able to show that a wider carapace did result in a crab better able to survive in polluted water. He also used his results to assert that Darwin was right; evolution happened gradually, for the carapaces varied on a continuum—there were no suddenly larger carapaces. This work also allowed him to demonstrate his other conviction, that science was best when it was subjected to mathematical analysis. With the crab carapaces varying in a continuous fashion, differences had to be subjected to statistical tests. This was Weldon and Pearson's forte, as well as being what they saw as good science. Following in the footsteps of Darwin's cousin Galton, Weldon was sure that science would advance only as long as it was subjected to the rigorous application of math. Weldon was one of a class of biologists calling themselves biometricians.
Bateson, on the other hand, was math-phobic. He admitted to requiring extensive tutoring in math to qualify for his studies and then quickly forgetting it all. Statistical applications appalled him. Instead, he did his first work out of doors, traveling to the Central Asian steppes. There he studied marine invertebrates found in lakes derived from the Aral Sea. Each lake, conveniently enough, had different salt content, varying on a nearly continuous curve. The invertebrates, on the other hand, exhibited discontinuous variations. This supported his belief that the evolutionary change had to be saltatory.
Battle lines were drawn. Biology was to be, for the next few decades, split between gradualists and saltationists, bio-metricians and naturalists. In hindsight, much of what they fought over was not exclusionary. The dispute nonetheless slowed progress of evolutionary thought. This was due not a little to the vituperation between Bateson and Wel-don. The two men were increasingly critical of each other, reviewing each other's work in more and more personal terms. They even got to the point of suppressing each other's publications.
The discovery of Mendel's work added fuel to the flames. Bateson read the paper after reading reports on experiments repeating the work by three Europeans (including de Vries) in corn, chickens, and mice. Somewhat incongruously, considering his distaste for mathematical methods, Bateson quickly accepted Mendel's conclusions. After all, to his mind, the particulate nature of Mendel's elements proved that evolution occurred in jumps.
Weldon did not agree, and he responded by attacking Mendel's experimental method. He protested that Mendel had never measured how green was green, and that pea plant seeds showed a continuous variation between yellow and green. Mendel's divisions were impossible to judge, he asserted, and it was too bad he hadn't made an attempt to describe them in a quantitatively accurate fashion.
Bateson struck back, defending Mendel and nonquantitative biology in general. Weldon and Pearson returned fire, criticizing Bateson's own experiments, finding his methods woefully imprecise. Of a paper by Bateson on crossing the hairy and nonhairy variants of the white campion flower, he asked, How hairy was hairy? He went on to bemoan imprecise methods in general, writing "the accumulation of records, in which results are massed together in ill-defined categories of variable and uncertain extent, can only result in harm."
All this general malignity came to a head in a meeting of the British Association of Zoology in 1904. Bateson had just been made president, but Weldon spoke first. Bateson replied. It was a rowdy debate. Pearson rose to call for a truce over the next three years so that the differences could be worked out. Finally, T. R. Stebbing, a cancer scientist, rose to bring the meeting to an end. A witness describes his speech as follows:
In a preamble he deplored the feelings that had been aroused, and assured us that as a man of peace such controversy was little to his taste. We all began fidgeting at what promised to become a tame conclusion to so spirited a meeting, especially when he came to deal with Pearson's suggestion of a truce. But we need not have been anxious for the Rev. Mr. Stebbing had in him the makings of a first rate impresario. "You have all heard," said he, "what Professor Pearson has suggested" (pause), and then with a sudden rise of voice, "But what I say is: let them fight it out!"
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