But genius is always impatient of its harness; its wild blood makes it hard to train.
 Oliver Wendell Holmes Sr., The Professor at the Breakfast-Table (1860). copy citation

Context

“Talent is a docile creature. It bows its head meekly while the world slips the collar over it. It backs into the shafts like a lamb. It draws its load cheerfully, and is patient of the bit and of the whip. But genius is always impatient of its harness; its wild blood makes it hard to train. Talent seems, at first, in one sense, higher than genius,—namely, that it is more uniformly and absolutely submitted to the will, and therefore more distinctly human in its character. Genius, on the other hand, is much more like those instincts which govern the admirable movements of the lower creatures, and therefore seems to have something of the lower or animal character.” source