There is also a class of men who, although disharmonious as the criminal and the insane, are indispensable to modern society. They are the men of genius. They are characterized by a monstrous growth of some of their psychological activities. A great artist, a great scientist, a great philosopher, is rarely a great man. He is generally a man of common type, with one side over-developed. _As a combination of traits_. Galton (6) thought of genius as that which qualifies a person for eminence, and he believed that achieved eminence must be founded on a combination of no less than three essentials. He wrote: By natural ability I mean those qualities of intellect and disposition which urge and qualify a man to perform acts that lead to reputation. I do not mean capacity without zeal, nor zeal without capacity, nor even a combination of both of them, without an adequate power of doing a great deal of very laborious work. But I mean a nature which, when left to itself, will, urged by an inherent stimulus, climb the path that leads to eminence, and has strength to reach the summit . . . one which, if hindered or thwarted, will fret and strive until the hindrance is overcome, and it is again free to follow its labour-loving instinct. It is almost a contradiction in terms to doubt that such men will generally become eminent. Again, Galton says: We have seen that a union of three separate qualities--intellect, zeal, and power of work--are necessary to raise men from the ranks. Lehman (9) has recently expressed this same idea, as a result of a statistical study of the most productive years of intellectual workers: Indeed, it is doubtful that genius is solely the fruit of any single trait. It is the belief of the writer that the fruits of genius are, on the contrary, a function of numerous integers, including both the personal traits of the individual worker, environmental conditions that are not too hostile, and the fortunate combination of both personal traits and external conditions. _As quantitative_. Galton (6) was the first to place the study of genius on the basis of quantitative statement, so that comparisons might be made and vertifications be effected. Galton formulated the theory that genius (great natural ability) is nothing more nor less than a very extreme degree in the distribution of a combination of traits--"intellect, zeal, and power of working"-- which is shared by all in various "grades" or degrees. Reasoning thus, Galton applied for the first time in human thought the mathematical concepts of probablity to the definition of genius.