Wollte man die schärfste Bestimmung des Artbegriffes in Anwendung bringen, nach welcher zu einer Art nur jene Individuen gehören, die unter völlig gleichen Verhältnissen auch völlig gleiche Merkmale zeigen, so könnten nicht zwei davon zu einer Art gezählt werden.
If one wanted to apply the strictest definition of the species concept, according to which only those individuals belong to one species which under entirely identical circumstances also exhibit entirely identical traits, then no two of them can be counted as belonging to one species.
species = Art Mendel is using a species criterion here that has become uncommon today and according to which species may be distinguished by differences with respect to a single trait, as long as that difference is exhibited under “entirely identical circumstances”; see Staffan Müller-Wille and Vitezslav Orel, “From Linnaean Species to Mendelian Factors: Elements of Hybridism, 1751–1870”, Annals of Science 64 (2007), 171–215. A possible contemporary source for this definition may be Stephan Endlicher and Franz Unger, Grundzüge der Botanik (Vienna: Gerold, 1843), p. 403–404 (thanks to Robert Meunier for this reference). Mendel studied with Unger and possessed a copy of this popular textbook, that he annotated; see Hugo Iltis, Gregor Johann Mendel: Leben, Werk und Wirkung (Berlin: Springer, 1924), p. 67. For an application of the species concept that Mendel develops in this sentence, see p. 8, s. 18.