The last decade of the nineteenth century and the first few decades of the twentieth century was a volatile time in the history of genetics. Mendel’s paper, though written in the mid-nineteenth century, had slipped into the ether until the triumverate of deVries, Correns, and Tschermak almost simultaneously discovered it in 1900. Most of the writings before their discovery did not know what to make of trait variation, especially whether there were some unknown or unknowable substances involved in facilitating trait variation. Before pressing the Bateson and Pearson debate, it is important to gain a clearer understanding of what was happening at the same time. The sole purpose of this section is to place the debate within a historical context.
The discovery of Mendel’s paper in 1900 by deVries et al is a defining moment in the biological sciences. Not only was it an important discovery for the development of genetics but also it was an important first-step toward the synthesis of Mendelian genetics with Darwinian evolutionary theory that occurred in the 1920s and 1930s, beginning with R.A. Fisher’s (1918) seminal paper “The Correlation Between Relatives on the Supposition of Mendelian Inheritance.”
H. F. Roberts says the following about the discovery of Mendel’s laws:
The year 1900 marks the beginning of the modern period in the study of heredity. Despite the fact that there had been some development of the idea that a living organism is an aggregation of characters in the form of units of some description, there had been no attempts to ascertain by experiment, how such supposed units might behave in the offspring of a cross. In the year above mentioned the papers of Gregor Mendel came to light, being quoted almost simultaneously in the scientific contributions of three European botanists, DeVries in Holland, Correns in Germany, and VonTschermak in Austria. (Roberts 1929, p. 286)
One cannot overstate the significance of this discovery. Before 1900, some were inclined to accept the notion that an organism was composed of something like units or factors of heredity, but no attempt had been made to test this hypothesis. In addition, there had not been any attempt to explain what these ‘units’ are. The re-emergence of Mendel, however, gave new hope to the hypothesis that there exist ‘units’ of heredity in individual organisms that may cause variation.
An unusual twist in the discovery of Mendel’s paper was that deVries et al. did not draw conclusions from reading Mendel’s paper. They had developed, on their own, experiments that provided evidence for the segregation of characters. Each of them saw their own work as laying the foundation for a theory of heredity. Only later after having performed the appropriate experiments and coming to almost identical conclusions did they each uncover the fact that Mendel had anticipated their experiments and conclusions some thirty years earlier. The verdict is still out as to whether the three had independently discovered these findings, or whether they had read Mendel before performing their experiments (particularly in the case of deVries). Nonetheless, the importance of the great triumverate and the discovery of Mendel’s paper forever changed science. Genetics had been born.
Before the discovery of Mendel’s paper and the work of deVries, Correns, and Tschermak, several natural scientists were working on the problem of inheritance. Two of the people involved were William Bateson and Karl Pearson. Bateson and Pearson’s work represents an important first step in solving the problem of inheritance. One such problem was whether there were Mendelian ‘factors’ or ‘units’ of heredity.
The odd fact about Mendel’s theory was that it postulated the existence of ‘factors’ or ‘units’ of heredity not visible to the naked eye (See §9 of Fisher's (1965) "The Reproductive Cells of the Hybrids"). Even with the proper scientific equipment, no geneticist at the time could observe Mendelian ‘factors’. These ‘factors’ were inferred to exist from the observation of variation in individual organisms. Perhaps T.H. Morgan characterizes the inference best in his paper “What are ‘Factors’ in Mendelian Explanations?” Morgan writes, “We work backwards from the facts to the factors, and then, presto! explain the facts by the very factors that we invented to account for them” (1909, p. 365). Morgan is skeptical of how Mendelians came to such a conclusion. Morgan is also sceptical of the existence of Mendelian 'factors' during the early part of his career (See Vicedo 1990 for more commentary on Morgan's epistemological views). This intuition is confirmed when Morgan later says, “the success of the method as a ready means of explanation does not, in my opinion, justify the procedure" (Morgan 1909, p. 366). The procedure postulates factors based upon the facts gleaned through experimentation. Even though such experimentation is successful, the success of the method does not justify the procedure. Such distrust of speculative reasoning abounded in Morgan’s early work. Gilbert points to Morgan’s belief that cytoplasm is the carrier of hereditary traits for the distrust of speculative reasoning, and Mayr blames Morgan’s fear of preformationism for his distrust (See Gilbert 1978, pp. 307-351 and Mayr 1982). Morgan was more convinced of empirical data drawn from experiments primarily because of his background in embryology (cf. Vicedo 1990, p. 299).
Mendelian ‘factors’ during the early part of the twentieth century suffer from the same flaw as atoms do. No one is able to observe atoms and verify their existence like someone may be able to do with a chair or table. Atoms are so tiny that even with the aid of specialised equipment they are empirically unobservable. None of our sensory modalities possesses the ability to perceive atoms. We may be able to observe objects, which are made up of atoms, but no object of experience satisfies the content – atom. By experience, I mean waking experience visible to the naked eye without the assistance of any physical device designed to maximize one of the five senses. For a definition of the use of scare quotes when referring to terms and expressions (cf. Austin's Sense and Sensibilia 1940).
Analogously, ‘units’ of heredity are unobservable. No objects of our normal waking experience satisfy the term – unit of heredity – in the same way that a chair satisfies the term ‘chair’. When we cannot observe certain things in the same way as observing chairs in a room, we will call this the verifiability problem of nonveridical theoretic objects.We can define this problem as the inability to visually, auditorally, or kinesthetically confirm the existence of objects postulated in a scientific hypothesis.