Wir müssen es daher als nothwendig erachten, dass auch bei Erzeugung der constanten Formen an der Hybridpflanze vollkommen gleiche Factoren zusammenwirken.
We therefore have to consider it as necessary that, also in the generation of constant forms on the hybrid plant, completely identical factors act together.
constant forms = constante Formen See p. 15, s. 7.
on the hybrid = an der Hybridpflanze See p. 10, s. 10.
factors = Factoren Mendel is taking over Gärtner’s terminology here who addressed the two parties in hybridisation as “Faktoren”; see e.g. arl Friedrich Gärtner, Versuche und Beobachtungen über die Bastarderzeugung im Pflanzenreich (Stuttgart: Hering, 1849), p. 186. As Robert C. Olby has emphasized, Gärtner used the term to refer to the species hybridized, rather than underlying dispositions for particular traits (Origins of Mendelism, 2nd edition, Chicago: University of Chicago Press, 1985, p. 33).
necessary = nothwendig Bateson has “essential”, Sherwood “inevitable”. Mendel is clearly deducing this statement from the statement made in the previous sentence. It provides the hypothesis to be tested by the back-crossing experiments that are the subject of this section. The deduction hinges, though, on whether the plant varieties crossed can indeed be addressed as “pure species”.