 Yet, the experimental control of biological material and conditions that make such a mapping possible also provided the Mendelian researchers of the early twentieth century a means to investigate the genotype-as-material-constituents (see  section 3).  Indeed, Johannsen’s conception of the genotype as a class of organisms sharing stable genotypic constituents already pointed that way. However, in taking up that direction of research what was left unaddressed was the relevance for understanding heredity in naturally varying populations of phenotype-to-genotype mapping and investigations of the constituents of genotype using Mendelian methods. Johannsen (1911) does not address those issues (which are returned to in  section 5),  but he does point to several other concerns about the concepts, methods, and implications of the genotype conception of heredity. These follow.