 A major theme that crystallized in this history of investigation is the distinction between epigenesis and preformation (see the entry on  theories of biological development).  Proponents of epigenesis claimed that heterogeneous, complex features of form emerge from homogeneous, less complex embryonic structures through interactive processes. Thus, an explanation of the ontogeny of these form features requires accounting for how the interactions occur. Proponents of preformation claimed that complex form preexists in the embryo and “unfolds” via ordinary growth processes. An adequate explanation involves detailing how growth occurs. Although preformation has a lighter explanatory burden in accounting for how form emerges during ontogeny (on the assumption that growth is easier to explain than process interactions), it also must address how the starting point of the next generation is formed with the requisite heterogeneous complex features. This was sometimes accomplished by embedding smaller and smaller miniatures ad infinitum inside the organism  (Figure 1).  Epigenetic perspectives were often dependent on forms of teleological reasoning (see the entry on  teleological notions in biology)  to account for why interactions among homogeneous components eventually result in a complex, integrated whole organism. Though nothing prevents mixing features of these two outlooks in explaining different aspects of development, polarization into dichotomous positions has occurred frequently (Rose 1981; Smith 2006).