He did this calculation a number of times, making various assumptions. In 1862 his estimate was between 20 and 400 million years; but in 1866 he reduced the top estimate to 100 million years, and attacked Darwin and Lyle for not taking notice of his calculation. We know Darwin was anxious and worried that this would not be long enough to allow for evolution. Thomas Henry Huxley remarked that Kelvin's calculations were good, but his assumptions were wrong. In 1897 Kelvin did the calculation one last time, and came up with 20 to 40 million years. This, of course, would definitely not be long enough to allow for evolution. Oliver Heaviside also disagreed, and proposed an alternative geothermal model. Both Kelvin and Heaviside were eventually shown to be wrong.