In this paper, the main points of Lamarck's and Darwin's theoretical conceptual schemes about evolution are compared to those derived from 15 years old students' explanations of evolutionary episodes. We suggest that secondary students' preconceptions should not be characterized as "Lamarckian", because they are essentially different from the ideas that Lamarck himself possessed. Most students in our research believed that needs directly impose changes on animal bodies in order to survive in a given environment and accepted the possibility of extinction whereas Lamarck believed that it was the effect of use or disuse that would produce changes on body structures and that species would transform but would not die out. We conclude that the relationship between secondary students' ideas and historical views on evolution should be treated more skeptically, given the differences in the historical, social and cultural contexts, and that instruction should focus on students' ideas of need-driven evolution as well as on the role of chance in the evolutionary process.