Tag Archives: Kuhn

Theories of Generation

by Tim Harding

Does Darwin successfully combine the two explanatory paradigms of inheritance, and growth and development?  If he does, is this a problem for Kuhn’s view of scientific change?

In this essay, I intend to show how Darwin’s hypothesis of pangenesis created a synthesis between the then competing paradigms relating to inheritance and development.  I think that this synthesis does present a problem for Kuhn’s view of scientific change, and that Popper’s account may actually be more relevant than Kuhn’s to certain aspects of this particular case study.

Before discussing Darwin’s theories, it is necessary to briefly describe the two paradigms that attempted to explain inheritance (how characteristics are transferred to offspring from parents), and growth and development (how the growth of offspring is organised).  These two paradigms were characterised by competing Hippocratic theories and preformationist theories (Verdnik, 1998:126, 157).

Before the advent of genetics, Hippocratic theories attempted to explain inheritance in terms of a blending of fluids extracted from all parts of both male and female bodies during intercourse.  It was thought that the characteristics of the offspring are determined by the relative amounts and strength of fluids from each part of the body of each parent (Verdnik, 1998:126).

On the other hand, preformationist theories held that the new mammalian offspring is already preformed in miniature, either within the egg of its mother (ovist preformationism) or in the semen of its father (animalculist preformationism).  Both of these types of theories incorporated emboîtment (encasement), which was the thesis that God created all future organisms in miniature, and that reproduction was just the growth and development of these miniatures (Verdnik, 1998:128).

Hippocratic theories were very good at explaining inheritance but very bad at explaining growth and development; whilst preformationist theories were the opposite – very good at explaining growth and development but very bad at explaining inheritance (Verdnik, 1998:126).  To give some examples, Hippocratic theories were unable to adequately explain phenomena such as the regeneration of freshwater polyps as observed by Trembley (Trembley 1744: 148); while preformationist theories were unable to adequately explain how the mating of a mare with a donkey produces a mule (Maupertuis 1745: 172).

Darwin came to his hypothesis of pangenesis, from a different direction – to fill a gap left in his theory of evolution, as published in his 1859 book The Origin of Species (Darwin, 1859).  Natural selection provided a mechanism for variation and eventual speciation, but it did not explain the inheritance of variation.  Without some way to explain the inheritance of characteristics acted on by natural selection, his theory would be incomplete.  Darwin’s breeding experiments on domestic animals (mainly pigeons) in the 1850s and 60s[1] were part of his attempts to complete his evolution theory (Bartley, 1992: 308).  He was attempting in these experiments to show just how quickly varying characteristics can be amplified by domestic breeding, and therefore how natural selection can operate (Verdnik, 1998:156).

evolution_peas

The Laws Of Inheritance & Pangenesis

Darwin called his explanation of inheritance ‘the hypothesis of Pangenesis’, which he published in Part II of Chapter XXVII of Variation Under Domestication (Darwin, 1875: 374-404).  However, he provides a more succinct description of this hypothesis in an earlier unpublished manuscript on pangenesis sent to Huxley in 1865:

‘Furthermore, I am led to believe from analogies immediately to be given that protoplasm or formative matter which is throughout the whole organisation, is generated by each different tissue and cell or aggregate of similar cells; – that as each tissue or cell becomes developed, a superabundant atom or gemmule as may be called of the formative matter is thrown off; – that these almost infinitely numerous and infinitely minute gemmules unite together in due proportion to form the true germ; – that they have the power of self-increase or propagation; and that they here run through the same course of development, as that which the true germ, of which they are to constitute elements, has to run through, before they can be developed into their parent tissues or cells. This may be called the hypothesis of Pangenesis’ (Olby 1963: 259).

Darwin further proposed that his hypothesis would not only account for inheritance, but also for development:

‘The development of each being, including all the forms of metamorphosis and metagenesis, as well as the so-called growth of the higher animals, in which structure changes, though not in a striking manner, depends on the presence of gemmules thrown off at each period of life, and on their development, at a corresponding period, in union with the preceding cells ‘ (Darwin, 1875: 403-404).

Through these mechanisms, Darwin proposed that inheritance and development were tied together – not only in the generation of offspring and early stages of embryonic life, but throughout the life of the organism (Bartley, 1992: 310).  By giving ‘gemmules’ the power to be modified throughout the life of an organism and then be transferred to the next generation, he argued that inheritance should be viewed as a form of growth (Bartley, 1992: 331).

By means of this single hypothesis, Darwin not only filled a gap in his theory of evolution, but whether he meant to or not, he created a synthesis between the then competing paradigms relating to inheritance and development.

The preceding discussion raises some significant philosophical issues.  How does it compare with Kuhn’s view of scientific change; and in particular, is Darwin’s paradigm synthesis a problem for the Kuhnian account?

According to Kuhn, ‘normal science’ operates within scientific paradigms[2] that not only determine which scientific theories are acceptable, but define scientific communities and even the areas of research undertaken (Kuhn, 1962: 10-11).

Kuhn says that in so far as he is engaged in normal science, the researcher is a solver of puzzles, not a tester of paradigms.

Paradigm-testing occurs only after persistent failure to solve a noteworthy puzzle has given rise to crisis.  And even then it occurs only after the sense of crisis has evoked an alternative candidate for a paradigm.  In the sciences, the testing situation never consists, as puzzle-solving does, simply in the comparison of a single paradigm with nature.  Instead, testing occurs as part of the competition between two rival paradigms for the allegiance of the scientific community (Kuhn, 1962: 144-145).

Kuhn notes that the schools guided by different paradigms are ‘always slightly at cross-purposes’ and that they ‘fail to make complete contact with each other’s viewpoints’, a phenomenon which describes as ‘incommensurability’ (Kuhn, 1962: 112, 148-150).  The failure of the adherents of Hippocratic and preformationist theories to focus on the strengths and weaknesses of both paradigms, and to engage in the testing of both paradigms, tends to support Kuhn’s notion of incommensurability.  No crisis occurred within each paradigm, because their adherents failed to focus on how well each paradigm explained both inheritance and development.

The formulation of Darwin’s hypothesis does not accord with the Kuhnian account.  He did not conduct his breeding experiments on domestic animals to test either the Hippocratic or the preformationist paradigms, but to fill a gap in his quite separate theory of evolution.  Similarly, his synthesis between the then competing paradigms relating to inheritance and development, was not done to resolve any crisis in these paradigms, but for a separate purpose.  A further problem for the Kuhnian account is that the scientific method Darwin used to formulate his hypothesis was essentially inductive, although this may also be a problem for the Popperian account.

Another aspect of the Kuhnian account is that the failure of experimental results to conform to the prevailing paradigm is seen as an ‘anomaly’, rather than as an instance of Popperian falsification.[3]  If scientists are confronted by anomalies, they will often devise ad hoc modifications in order to eliminate any apparent conflict.  Anomalies do not cause the abandonment of paradigm or theory unless and until the level of anomalies builds to a crisis where the prevailing paradigm or theory is replaced by a new one (Kuhn, 1962: 77-78).

After reading Variation Under Domestication, Francis Galton (a cousin of Darwin’s) arranged for a series of experiments to be conducted on rabbits initially housed in the Zoological Gardens of London and later at his Kensington home.  His intention was to demonstrate the transmission of ‘gemmules’ to succeeding generations via blood injected from one rabbit to another, using coat colour as a marker.  Galton ultimately found that not a single instance of induced variation of coat colour occurred in a total of 88 offspring from blood transfused parents, and in 1871 published his results in Nature (Brown, 2002: 290-291).  Although intended to verify Darwin’s hypothesis, it seems that Galton’s experiments provided a Popperian falsification instance.  Whether the purpose was verification or falsification, Galton’s experiments do not accord with Kuhn’s account of paradigm-testing discussed above.

In later editions of Variation Under Domestication, Darwin admitted in a footnote that he would have expected to find ‘gemmules’ in the blood, although their presence was not absolutely necessary to his hypothesis (Brown, 2002: 292).  I find Darwin’s response unconvincing, as he provides no alternative explanation as to how the ‘gemmules’ are transmitted from the parents’ somatic cells to the germ cells.  He made no real attempt to modify his hypothesis in response to Galton’s falsification of it.

I therefore conclude that whilst the Hippocratic and preformationist paradigms accord with Kuhn’s notion of incommensurability, Darwin’s synthesis of these two paradigms does not accord with the Kuhnian account of science.  Furthermore, Darwin’s failure to make an ad hoc modification to his hypothesis after the discovery by Galton of an anomaly, as would be expected under the Kuhnian account, supports a Popperian rather than a Kuhnian account of the scientific process in this case.

References:

Bartley, M.M. (1992) Darwin and Domestication: Studies on Inheritance. Journal of the History of Biology, Vol 25, No. 2 pp.307-333.

Browne, J. (2002) Charles Darwin – The Power of Place, Volume II of a Biography. London: Pimlico.

Darwin, C. (1859) The Origin Of Species By Means Of Natural Selection, Or The Preservation Of Favoured Races In The Struggle For Life. 6th ed. 1873. London: John Murray.

Darwin, C. (1875) The Variation of Animals and Plants Under Domestication, Vol II London: John Murray.

Kuhn, T.S. (1962) The Structure of Scientific Revolutions 3rd ed. Chicago: University of Chicago Press.

Maupertuis, P. (1745) The Earthly Venus in Verdnik, D. (ed.) (1998) Thinking about Science. Study Guide and Readings. Churchill: Monash Distance Education Centre.

Olby, R.C. (1963) Charles Darwin’s Manuscript of Pangenesis, British Journal for Philosophy of Science 1: 251-263.

Popper, K. (1959) The Logic of Scientific Discovery. rev. ed. 1968, in Verdnik, D. (ed.) (1998) Thinking about Science. Study Guide and Readings. Churchill: Monash Distance Education Centre.

Trembley, A. (1744) Memoires pour server a l’histoire d’un genre de polypes d’eau douce in Verdnik, D. (ed.) (1998) Thinking about Science. Study Guide and Readings. Churchill: Monash Distance Education Centre.

Verdnik, D. (ed.) (1998) Thinking about Science. Study Guide and Readings. Churchill: Monash Distance Education Centre.

 Endnotes:

[1] Darwin reported the results of these experiments in both The Origin Of Species and The Variation of Animals and Plants Under Domestication.

[2] Kuhn does not provide a concise definition of a scientific paradigm, but the Merriam-Webster Online dictionary defines it as ‘a philosophical and theoretical framework of a scientific school or discipline within which theories, laws, and generalizations and the experiments performed in support of them are formulated; broadly: a philosophical or theoretical framework of any kind.’

[3] Popper proposes falsifiability as the criterion of demarcation between scientific and non-scientific statements.  In other words, a theory is scientific if it is falsifiable (Verdnik, 1998:76).

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