“His act was rather that of a harmless lunatic than an enemy. We were not so new to the country as not to know that the solitary life of many a plainsman had a tendency to develop eccentricities of conduct and character not always easily distinguishable from mental aberration. A man is like a tree: in a forest of his fellows he will grow as straight as his generic and individual nature permits; alone, in the open, he yields to the deforming stresses and tortions that environ him.”