Eagleton is paraphrasing a letter from Lawrence to Edward Garnett:
You mustn’t look in my novel for the old stable ego of the character. There is another ego, according to whose action the individual is unrecognisable, and passes through, as it were, allotropic states which it needs a deeper sense than any we’ve been used to exercise, to discover are states of the same single radically unchanged element. (Like as diamond and coal are the same pure single element of carbon. The ordinary novel would trace the history of the diamond—but I say, ‘Diamond, what? This is carbon.’ And my diamond might be coal or soot, and my theme is carbon.) You must not say my novel is shaky—it is not perfect, because I am not expert in what I want to do. But it is the real thing, say what you like. And I shall get my reception, if not now, then before long. Again I say, don’t look for the development of the novel to follow the lines of certain characters. The characters fall into the form of some other rhythmic form, as when one draws a fiddle-bow across a fine tray delicately sanded, the sand takes lines unknown.
D. H. Lawrence (5 June 1914). Letter to Edward Garnett. In Aldous Huxley, ed. (1932). The Letters of D. H. Lawrence, pp. 198–199. London: Heinemann. Some punctuation added.
(The way that I found this was to guess that Eagleton had heavily paraphrased Lawrence, but that the most significant word, “ego”, might have appeared in the original. Searching for “lawrence ego” on Google Books, I found, as the third hit, a critical study by Doo-Sun Ryu which quotes the “now famous letter” and gives a reference.)