Given the following passage  "Evolution had less obvious applications to anatomy and morphology, and at first had little impact on the research of the anatomist Thomas Henry Huxley. Despite this, Huxley strongly supported Darwin on evolution; though he called for experiments to show whether natural selection could form new species, and questioned if Darwin's gradualism was sufficient without sudden leaps to cause speciation. Huxley wanted science to be secular, without religious interference, and his article in the April 1860 Westminster Review promoted scientific naturalism over natural theology, praising Darwin for "extending the domination of Science over regions of thought into which she has, as yet, hardly penetrated" and coining the term "Darwinism" as part of his efforts to secularise and professionalise science. Huxley gained influence, and initiated the X Club, which used the journal Nature to promote evolution and naturalism, shaping much of late Victorian science. Later, the German morphologist Ernst Haeckel would convince Huxley that comparative anatomy and palaeontology could be used to reconstruct evolutionary genealogies.",  answer the following question. Note that the answer is present within the text.  Question: What did Darwin theorize?
Answer:
Evolution