 To put that in another way, the influence of factors that are identical for all members of a species cannot be studied through Mendelian crosses. The genotype-conception of heredity, by centering on genotypic differences associated with phenotypic differences, shifted attention away from the species-typical aspects of the germ cells and subsequent development. Mendelian analysis focused on differences over similarity, even though both aspects were included in then prevailing conceptions of heredity (Sapp 1987). (Similarity was part of heredity in the sense that, for the eye color of some flies to differ from the rest of the population, the initial cell or zygote of the fly has to be able to develop into an organism that has eyes with color.) 