At the beginning of the twentieth century, the Italian cytologist Paolo Della Valle developed a theory of instable chromosomes (teoria dei cromosomi labili). He radically criticized the so-called Sutton–Boveri hypothesis (Martins and Martins, Genetics and Molecular Biology, 22:261–271, 1999), focusing on numerical constancy in the species and individuality. On the basis of bibliographical review and personal observations, he maintained that the chromosomes were neither stable bodies, nor permanent structures, but transitory cellular materials, resulting from the periodical re-arrangement of the chromatin (...) during the cell division. German and English-speaking biologists reacted. The paper shows some content of the argumentations used by Thomas H. Montgomery and especially Edmund B. Wilson. The discussion was characterized by the same data which is interpreted by different scholars in different ways. And the point is that no one of them had the decisive test to demonstrate his own point of view. Wilson simply invoked on his behalf a certain ‘common sense’, defending at least a ‘high degree of constancy’. The debate waned along with the reception of Morgan’s chromosome theory of heredity, but only the advent of molecular biology definitively stated the nature of chromosomes as permanent structures of the cell. (shrink)