It is now received that de Vries’ work is better characterised as the incorporation of Mendel’s work into his project of the theory of pangenesis. However, there is still lack of a systematic evaluation of de Vries’ incorporation. It is unclear in what sense de Vries’ incorporation was significant for the origin of genetics. Nor is it very clear what role Mendel’s work played in de Vries’ incorporation. Would there have been a Mendelian theory of heredity in the first decade of the twentieth century if Mendel had not ever written or published his paper on Pisum? This chapter examines the significance of de Vries’ work and its relation to Mendel’s work. Firstly, I argue that Mendel’s work was indispensable for de Vries’ conceptualisation and theorising. Secondly, I show that de Vries’ unsuccessful incorporation was still important for its introduction of Mendel’s work to the study of heredity and the proposal of the law of segregation.