Source: Elbert Hubbard, As quoted from Electrical Review (c. 1895) without further attribution in The Search for the North Pole (1896) by Evelyn Briggs Baldwin, page 520.

Complete Sentence: Genius is often only the power of making continuous efforts. The line between failure and success is so fine that we scarcely know when we pass it — so fine that we are often on the line and do not know it. How many a man has thrown up his hands at a time when a little more effort, a little more patience, would have achieved success. As the tide goes clear out, so it comes clear in. In business sometimes prospects may seem darkest when really they are on the turn. A little more persistence, a little more effort, and what seemed hopeless failure may turn to glorious success. There is no failure except in no longer trying. There is no defeat except from within, no really insurmountable barrier save our own inherent weakness of purpose.