Detailed autographed letter signed in full by the father of Social Darwinism, explaining to a correspondent why he cannot visit, in part: "These engagements will make it impracticable for me to accept your invitation. Moreover even in the absence of any such difficulties the prospect you hold out is alarming...as I am suffering a good deal from head symptoms." 2 pages, slightly foxed with a tiny, red-inked annotation by the recipient on the upper left corner. 8vo. Bloomsbury Square, London, July 4, 1864.