 In 1899, American biologist William Morton Wheeler suggested that there are just two different kinds of thinkers. Some see change and process, while others see stability. Heraclitus, Aristotle, physiology, and epigenesis characterize one way of looking at the world, while Parmenides, Plato, morphology, and preformationism characterizes another. These are, Wheeler felt, stable and persistent classes, with just the nature and details of their differences changing over time. Yet by the end of the nineteenth century, he argued, neither a strict preformationist nor a strict epigeneticist should prevail. Rather he called for a middle ground, for: