Having once advanced in his investigations from metamorphosis in the parts of the single plant to metamorphosis among different representatives of single plant species, Goethe had to take only one further, entirely decisive, step in order to recognize how every member of the plant kingdom is the manifestation of a single formative principle common to them all. He was thus faced with the momentous task of preparing his spirit to think an idea from which the plant world in its entire variety could be derived.