Why did William Thomson oppose Charles Darwin’s theory of evolution?

William Thomson, later known as Lord Kelvin, opposed Charles Darwin’s theory of evolution primarily because he believed it conflicted with the established laws of physics.1

Darwin and other uniformitarians argued that biological and geological changes occurred over an "incalculable" amount of time, assuming that natural forces had always operated at their current rates. Thomson challenged this by applying thermodynamics to calculate the age of the Earth and the Sun. Assuming the Earth was an inert body cooling from an initially molten state, he calculated that it could not be more than a few hundred million years old—a timeframe he later revised downward to between 20 and 40 million years.123

Because this duration was far shorter than what evolutionists required for natural selection to produce the diversity of life, Thomson insisted that biological theory must conform to the constraints of physical science. His calculations were later proven inaccurate due to the discovery of radioactivity, which demonstrated that the Earth generates its own internal heat, thereby invalidating his assumption that the planet was simply cooling down.123