The truth is, the importance of imaginative power in the equipment of a great scientist is often underestimated. Exact and watchful vision is the first necessity; but it does not go far, or not farthest, except as it has behind it the thoughts that wander through eternity, the vast and questing genius that is perpetually on the lookout for causes and explanations and is eager to evolve theory from the sure and substantial but inanimate basis of fact. Even Thoreau almost deplores his intense preoccupation with the fascinating business of observing: ‘Man cannot afford to be a naturalist, to look at Nature directly, but only with the side of his eye. He must look through and beyond her. To look at her is fatal as to look at the head of Medusa. It turns the man of science to stone. I feel that I am dissipated by so many observations.... I have almost a slight, dry headache as the result of all this observing.’[442] Theory, speculation, must be perpetually checked and restrained by the precision of systematic logic, but the accurate eye and the careful finger need to be supplemented by the eternally active mind. As to the activity of Darwin’s mind there can be no question whatever. He not only saw, but he thought incessantly. If you compare the Beagle Journal with the Journal of Thoreau, you see at once how much more quick and ready the English naturalist is with speculation and conjecture. The smallest fact is apt to set him off on a train of theory, where Thoreau simply records, or possibly compares, and passes on. How significant is the brief comment of Asa Gray, in regard to some botanical point which as a specialist in that line he should have been the first to develop: ‘That is real Darwin. I just wonder you and I never thought of it. But _he_ did.’[443] And he not only thought himself, he had the rarer and more valuable faculty of making others think. His mind was so intense and so magnetic in its constant activity that all those who came into contact with it were impelled and fired to work double on speculation of their own. ‘You stimulate my mind,’ says Gray again, ‘far more than any one else, except, perhaps Hooker.’[444] On this point of intellectual fertility, as on his other scientific qualifications, it is most interesting to hear Darwin himself. The mental activity was present early and late, and it does not appear that the exuberance of youth especially emphasized it or that it tended to increase with the later desire to develop and elaborate his special theories. He himself says in the Autobiographical sketch: ‘I am not conscious of any change in my mind during the last thirty years, excepting in one point: ... I think that I have become a little more skillful in guessing right explanations and in devising experimental tests; but this may probably be the result of mere practice, and of a larger store of knowledge.’[445]