Criminal sociology is rich in data as to the anatomical and physiological sides of the moral delinquent; Lombroso, Ferri, Marro, and others of what may be called the radical school of writers, pushing their theories of pathology and atavism to the front, often with a too heedless disregard of every obstacle; and others of the conservative class—Mantegazza, Colajanni, and their many colleagues of the sociological and psychological coterie—endeavoring, on the contrary, to account for every species of crime on the ground of economic and social conditions.

While the latter are as often right as the former, they find greater difficulty in proving their deductions; the "yardstick method" not applying to matters of a spiritual nature as it does to cranial measurements, and physical deformities. Lombroso once replied to his critics, who complained, not without cause, of the meagreness of his descriptive details along psychic lines, by impatiently exclaiming that he did not think it "necessary to prove that the sun shines;" meaning that the psychic element in human life is self-evident as sunlight is to the senses. And yet, by that light alone, which others besides the great Italian have ignored, or only casually treated, are the spots of recalcitrancy alone detected on the moral landscape of character, and the mind illumined in its search for those biological data which the great Turin criminologist, more than any other, delighted to discover.