For a very sophisticated delineation of nineteenth-century systematists' varied logical definitions of species, see Peter Stevens, The Development of Biological Systematics: Antoine-Laurent de Jussieu, nature, and the natural system (Columbia UP, 1994). I am not certain that I agree with the tone of Blumberg's comment in note 3 to section 2, that the desire for necessary and sufficient conditions in the definition of species is more typical of Mendel's background as a physicist than of a natural historian. As Stevens painstakingly and incisively demonstrates, plant taxonomists worried enormously (though in many ways not very productively, since they were worrying about pretty much unanswerable philosophical questions) about the limits and essential nature of species; they had to, since the logical question bears on the Big Question of what nature really is. Mendel here is taking the position, in Stevens's terms, of limital nominalism, that is, he believes that species at their cores are real things, but that the boundaries between one and another are arbitrary and set for our convenience.
Abigail Lustig, Department of History, Office for the History of Science and Technology, Univ. of California, Berkeley -- firstname.lastname@example.org