
The first sentence is often quoted in isolation http://www.conservapedia.com/Charles_Darwin, with the suggestion that Darwin is saying that his speculations concerning evolution "run quite beyond the bounds of true science." In fact, as the context makes clear, Darwin is referring to his speculations concerning the geographical ranges of genera with few species.
Other letters, notebooks, journal articles, recollected statements
Source: Letter http://www.darwinproject.ac.uk/letter/entry-2109 to Asa Gray, 18 June 1857