Summary — Napoleone Colajanni (1847-1921), well known as a leftist politician and meridionalist thinker, has been little studied as a positivist and has been rather neglected as Cesare Lombroso’s chief opponent. Well aware of the political and social consequences of the lombrosian anthropology, Napoleone Colajanni got involved, with all his might, in the bringing down of Lombroso’s theoretic structure, an indispensable step towards victory in his battles as politician and meridionalist. The second reason which explains why Napoleone Colajanni is overlooked as Lombroso’s adversary lies in the incompleteness of the enterprise of historiographic revision in Italian positivism. While the historiography of positivism does not merely conceive it as nothing but a negation of neo-idealist philosophy, it does not yet see in it a true conception of the world, a protean culture with currents such as the Lombroso-Colajanni antagonism — an antagonism revealing the existence of a positivist conservative culture and a positivist progressist culture. Last but not least, the opposition between Lombroso and Colajanni shows how the validity of social Darwinism was already called into question in the last century. La sociologia criminale by Napoleone Colajanni is thus a warning against the eugenicist and racialist implications in L’uomo delinquente by Cesare Lombroso. Napoleone Colajanni takes care to denounce the scientific prejudices of Lombroso’s criminal anthropology and to criticize its theorical grounds. The first level of his criticism consists in refusing to recognize with Lombroso that science may allow one to prove the existence of anatomical or biological criterion for discriminating between man and delinquent. He goes on to show that no geographical or biological determinism can determine a human hierarchy from the morphologic point of view, a hierarchy capable of expanding itself into the psychological and moral sphere. These types of geographical or biological fatalism are leaven of racism. Napoleone Colajanni’s originality lies finally in grounding his criticism of racism on a definition of the relationship between sociology and biology. While rejecting the separation as much as the confusion between both disciplines, he tries to prove that the specificity of sociology is to understand man as dependent on the law of altruism at the root of all human societies rather than on the law of egoism which dominates the reign of nature.