of instinctive insight into the method of attack of any biological problem, however unfamiliar to him, while he rigidly controlled the fertility of his mind in hypothetical explanations by the no less fertility of ingeniously devised experiment.’[456] In regard to this matter of speculative freedom and the tendency to let imagination run wild, it is interesting to watch Darwin’s comments on the general methods of others. The excess of abstinence he indeed deplores, recognizing that a man’s extreme caution may prevent him from theorizing enough: ‘How many astronomers have labored their whole lives on observations, and have not drawn a single conclusion.’[457] But the danger on the other side is so great and so ruinous that it cannot be enough insisted on, and indeed at times it makes all generalization suspicious and almost a thing to be eschewed: ‘I look at a strong tendency to generalize as an entire evil.’[458] At any rate, the theorist must never forget the subjective element, that his preconceptions and prejudices are apt to warp his judgment and distort his vision, till the keenest of observers and the sanest of thinkers may go astray: ‘the firmest conviction of the truth of a doctrine by its author seems, alas, not to be the slightest guarantee of truth.’[459] The profit and the lesson of all which must be constantly borne home to oneself: ‘When I think of the many cases of men who have studied one subject for years, and have persuaded themselves of the truth of the foolishest doctrines, I feel sometimes a little frightened, whether I may not be one of these monomaniacs.’[460]