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Disagreements among bacteriologists about the role in bacterial motility of the tail-like structures called flagella began in the late nineteenth century. Whereas many influential figures in bacteriology argued that flagella were motor organs, others saw them as useless appendages. The main evidence in favor of flagella as motor organs was only that they were seen on most motile species and were not found on nonmotile ones, yet this conclusion had come to be accepted by the overwhelming majority of bacteriologists before the 1930s. A 1946 paper by Adrianus Pijper challenged this consensus, reviving an old theory on the basis of new evidence. The argument on which the accepted view was based, Pijper asserted, was bad logic, for it was just as likely that flagella could be a product of motion as its cause. His observations suggested that the flagella must be merely twirls of polysaccharide material that, because of the active gyration of the cell body, spun out behind the cell as it swan. Further, his claim that movement is accomplished by undulations of a flexible cell body and that the flagella are a nonessential product of motility suggested a simple explanation for the existence of very motile bacteria without flagella.

Since flagella were taken to be an important feature for purposes of classification, Pijer`s claims called into question the fundamental taxonomy of bacterial species. Not only were the flagella not motor organs, according to Pijper`s argument, they were ephemeral and nonessential, so that all attempts to use them in taxonomic schemes were doomed to hopeless confusion. In addition, Pijper`s claims brought up another important issue whereas most of those interested in bacterial motility in the 1940s were asking whether electron microscopy(EM), the ultracentrifuge, or other high-tech tools just being introduced into biology could provide insight into this problem, Pijper specifically argued that these techniques could not get at the essence of motility, and that priority should be given to observations made on living cells.

In light of later evidence that confirmed that flagella actually are the motor organs of bacteria, it may be tempting to conclude that Pijper was simply being irrational. But at the time of the controversy, the evidence was still ambiguous, and it is clear that a preexisting commitment to the power and prestige of the electron microscope on the part of Pijper`s opponents was at least as important in their opposition as was the strength of the evidence per se. Whereas the two camps disagreed about who had superior evidence, they disagreed because they held fundamentally differing assumptions about biology. Reflected and intensified by the personal stakes involved in, they have tied their careers to the fortunes of a particular experimental system.

The passage suggests that Pijper and his opponents most likely agreed that

Athe function of flagella cannot be determined conclusively
BIt is important to observe living bacteria in order to gather unambiguous data about bacterial motility
CFlagella can be found on most motile bacteria
DFlagella are not fundamental to taxonomic classification of bacteria
EFlagella can be formed in more than one way

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