Moreover, fortunately for the world, Darwin's own mind was essentially one of the inductive type. If a great deductive thinker and speculator like Herbert Spencer had hit upon the self-same idea of survival of the fittest, he might have communicated it to a small following of receptive disciples, who would have understood it and accepted it, on _a priori_ grounds alone, and gradually passed it on to the grades beneath them; but he would never have touched the slow and cautious elephantine intellect of the masses. The common run of mankind are not deductive; they require to have everything made quite clear to them by example and instance. The English intelligence in particular shows itself as a rule congenitally incapable of appreciating the superior logical certitude of the deductive method. Englishmen will not even believe that the square on the hypotenuse is equal to the squares on the containing sides until they have measured and weighed as well as they are able by rude experimental devices a few selected pieces of rudely shaped rectangular paper. It was a great gain, therefore, that the task of reconstructing the course of organic evolution should fall to the lot of a highly trained and masterly intelligence of the inductive order. Darwin had first to convince himself, and then he could proceed to convince the world. He set about the task with characteristic patience and thoroughness. No man that ever lived possessed in a more remarkable degree than he did the innate capacity for taking trouble. For five years, as a mere preliminary, he accumulated facts in immense variety, and then for the first time and in the vaguest possible way he--'allowed himself to speculate.' That brings us down to the year 1842, when the first notes of the 'Origin of Species' must have been tentatively committed to paper. It was in 1859 that the first edition of the complete work was given to the world. Compare this with the case of Newton, who similarly kept his grand idea of gravitation for many years in embryo, until more exact measurements of the moon's mass and distance should enable him to verify it to his own satisfaction.