The swamping argument is an objection against Darwinism made by Fleeming Jenkin. He asserted that an accidentally-appearing profitable variety cannot be preserved by natural selection in the population, but should be 'swamped' with ordinary traits. Later, population genetics helped to overcome this logical difficulty.
Jenkin published his article "The Origin of Species" in the North British Review in June 1867.
Darwin agreed that a variation originating in a single individual would not spread across a population, and would invariably be lost. In the fifth edition of On the Origin of Species he responded:
Darwin concluded that natural selection must instead act upon the normal small variations in any given characteristic across all the individuals in the population.
The swamping argument was secondary to Jenkin's central thesis. Jenkin asserted that the population-average of any characteristic of an organism could be modified by selection (natural or human), but only within certain definite bounds. He further asserted that once selective pressure was removed, the population would revert to its original condition. Jenkin then introduced the swamping argument to deny the possibility that an occasional monstrous individual could supply an escape from this state of affairs.