According to Professor Gustav von Niessl, a staff member of the school where Mendel taught, Mendel thought Darwin's theory was inadequate and "hoped that his own researches would fill this gap in the Darwinian system." (Iltis 1924). Callender (1988) discusses an often misinterpreted paragraph of Mendel's, concerning Gärtner's Transformation experiments.
"The success of transformation experiments led Gärtner to disagree with those scientists who contest the stability of plant species and assume continuous evolution of plant forms. In the complete transformation of one species into another he finds unequivocal proof that a species has fixed limits beyond which it cannot change. Although this opinion cannot be adjudged unconditionally valid, considerable confirmation of the earlier expressed conjecture on the variability of cultivated plants is to be found in the experiments performed by Gärtner." (Mendel 1866, p. 47)
Callender cites a popular interpretation, that Mendel was dissociating himself from Gärtner's position. He argues that Mendel clearly meant the opposite: he gave conditional acceptance to Gärtner's view. Both interpretations ignore the context in which the paragraph appeared. The "earlier expressed conjecture" presumably refers to the following paragraph:
"If one may assume that the development of forms proceeded in these experiments in a manner similar to that in Pisum, then the entire process of transformation would have a rather simple explanation. The hybrid produces as many kinds of germinal cells as there are constant combinations made possible by the traits associated within the hybrid, and one of these is always just like the fertilizing pollen cells." (Mendel 1866, p. 44)
Mendel was arguing that the laws of variability he developed for
Pisum could be applied to Gärtner's experiments to explain his results. Without committing himself to one view or the other, he proposed that his laws of variability were in accordance with Gärtner's observations. He referred to his result as the "law valid for
Pisum" but he clearly intended it to be generally applicable. The significance Mendel attached to constant hybrids amounted to a partial acceptance of Linnaeus's modified theory of special creation.
"If the compromise be considered complete, in the sense that the hybrid embryo is made up of cells of like kind in which the differences are entirely and permanently mediated, then a further consequence would be that the hybrid would remain as constant in its progeny as any other stable plant variety. (Mendel 1866, p. 42)
He had established that constant hybrids did exist, but his application of that result to the question of the source of actual forms was only tentative.