Lord Acton in Historical Essays and Studies, Chapter IX, at Online Library of Liberty.
"Her talents ripened successively and slowly. No literary reputation of this century has risen so high after having begun so late. The even maturity of her powers, original and acquired, lasted only thirteen years, and the native imagination was fading when observation and reflection were in the fulness of their prime."
See George Eliot, The Lifted Veil in Gateway to the Great Books (10 Vol., 1963) volume 3, and Middlemarch in Great Books of the Western World (second edition, 60 Vol., 1990) volume 46