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Abstract.
Fleeming Jenkin's swamping argument (1867) is re-examined in relation to subsequent criticisms of its assumptions. Jenkin's original argument purported to show that, under blending inheritance, natural selection could not operate on 'sports' or 'single variations'. A serious flaw in Jenkin's model was exposed in a forgotten paper in 1871. Darwin accepted Jenkin's 'flawed' conclusion, though he did not fully understand the argument. Both Jenkin and Darwin regarded the swamping argument as a barrier to evolution within a single lineage. A completely different interpretation of the phrase 'swamping argument', first put forward by Romanes in 1886, identified it with the problem of the role of free intercrossing in preventing speciation. The latter problem also underlies current debate about the possibility of sympatric speciation and is as serious under particulate as under blending inheritance. Jenkin's argument depended on the assumption of blending inheritance; when modified to remove the 'flaw' in his model, it ceased to present a barrier to the operation of natural selection within a lineage, provided that the mutation rate was high enough to maintain adequate genetic variability under blending.
In June 1867 the North British Review published an anonymous review of Darwin's Origin of Species, whose author was the electrical engineer Fleeming (pronounced 'Flemming') Jenkin.1 He produced three major objections to Darwin's theory. First, he argued that variability within species was confined within strict limits, so that transformation from one species to another was impossible: 'We all believe that a breeder, starting business with a considerable stock of average horses, could, by selection, in a very few generations, obtain horses able to run much faster than any of their sires or dams; ... but would not the difference in speed between each successive generation be less and less?' This was a standard anti-transformationist argument, articulated as follows by Lyell: 'There are fixed limits beyond which the descendants from common parents never deviate from a common type.' Second, Jenkin argued that natural selection would be ineffective in selecting a rare 'sport' because it would be swamped by backcrossing to the normal population. Third, he argued that the new energy physics with its emphasis on the finite nature of the Sun's heat had undermined the Lyellian uniformitarian assumptions which allowed endless time for natural...