“I need scarcely say that the beginning and maintenance of life on earth is absolutely and infinitely beyond the range of sound speculation in dynamical science.”

Encountering Directors interview (1969)

Antony Hewish Interview https://www.countercurrents.org/ziabari171012.htm (17 October, 2012)

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

Book III, Ch. IV, pp. 150-152.
Physics

"Heroic Reason", as translated by H. R. Hays, in Selected Writings of Juan Ramon Jimenez (1957) edited by Eugenio Florit, p. 231.
Context: Dynamic ecstasy is absolute romanticism, absolute heroism. And here I return to my point. From my point of view, after the catastrophe which we feel and think is universal, a catastrophe resulting from an excess of useless dynamism of useless progress, of useless realism, of useless technology, after this an unattainable democracy is to be reached through the conception and realization of a new romanticism.

Fellini on Fellini (1976) edited by Anna Keel and Christian Strich; translated by Isabel Quigly.
Variant: There is no end. There is no beginning. There is only the passion of life.

Vol. I, Part 1, [The Materialist Conception of History].
The German Ideology (1845/46)
Context: Where speculation ends — in real life — there real, positive science begins: the representation of the practical activity, of the practical process of development of men. Empty talk about consciousness ceases, and real knowledge has to take place. When reality is depicted, philosophy as an independent branch of activity loses its medium of existence. At the best its place can only be taken by a summing-up of the most general results, abstractions which arise from the observation of the historical development of men. Viewed apart from real history, these abstractions have in themselves no value whatsoever. They can only serve to facilitate the arrangement of historical material, to indicate the sequence of its separate strata. But they by no means afford a recipe or schema, as does philosophy, for neatly trimming the epochs of history. On the contrary, our difficulties begin only when we set about the observation and the arrangement — the real depiction — of our historical material, whether of a past epoch or of the present.

Introduction (p. vi)
Thorns (1967)

Source: 1930s, Propagation problems and impulse problems in dynamic economics, 1933, p. 197