 Fleeming Jenkin also argued that the time needed by Darwin’s theory was simply inadequate, supporting this claim by an appeal to the physical calculations of the probable age of the earth presented in publications by Fleeming Jenkin’s mentor, the Glasgow physicist William Thompson (Lord Kelvin, 1824–1907; Burchfield, 1975). On the basis of Thompson’s quantitative physical arguments, Fleeming Jenkin judged the time since the origin of the solar system to be insufficient for the Darwinian gradualist theory of species transformation to take place. Jenkin’s multi-pronged argument gave Darwin considerable difficulties and set the stage for more detailed empirical inquiries into variation and its causes. The time difficulties were only resolved in the twentieth-century with the discovery of radioactivity.