 Morgan and his group were adamant that neither the phenomenon of sex-linked inheritance nor the fact that non-disjunction would explain the observed deviations from this inheritance pattern were sufficient to prove the chromosome theory. A necessary element in their case was the cytology: The excessive X-chromosome was microscopically observable, and it was shown to be present exactly in those flies that failed to exhibit the criss-cross pattern. This is how the vera causa ideal was met: The chromosomes as carriers of the genes not only explained the phenomena, their existence was also supported by evidence other than the fact that they explained the phenomena. That’s how they earned their place in biology’s ontology.