Es konnte jedoch auch ein anderer Umstand dazu beitragen, dass die Resultate trotz der sorgfältigsten Beobachtung und Unterscheidung schwankend und unsicher wurden.
Another circumstance, however, could have contributed to the results becoming fluctuating and uncertain, despite the most careful observation and distinction.
circumstance = Umstand See p. 10, s. 13.
could have contributed = konnte … dazu beitragen Konnte is not subjunctive (which would be könnte), but past tense. In this and the following two sentences, Mendel sticks to the past tense, indicating that he is critically examining how Gärtner had set up his actual experiments. It is only after , that he switches to the present tense again and generalises.