When the human being puts forth a force in himself which is considerable but acts normally, we call it talent; when it is abnormal in its working we call it genius. It would seem, therefore, that genius is in reality some imperfect step in evolution by which mankind in its most vigorous and forward individuals is attempting to develop a faculty which the race as a whole is not strong enough as yet to command or to acclimatise. As always happens in such a movement, there is a considerable irregularity in the working of the new phenomenon. Sometimes Nature seems to prepare by heredity for these its new experiments & fine flowers of humanity.
Ref: Essays Divine and Human