Guido Castelnuovo is one of the mathematicians who gave eminence to the school of mathematics of Rome. The many twines between his life, the history of the country, the history of the city, and the developments of mathematics in Italy are the object of this paper. We consider the period between the annexation of Rome and the beginning of the first world war when the switch between Castelnuovo's interests in algebraic Geometry and his later interests in statistics, probability, and teaching

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... red. We examine, under the light of the above mentioned twines, the reasons for this switch.