ABSTRACT
A human seems like a more advanced animal than our ancient fish-like ancestor. And that ancestor seems more advanced than its single-celled ancestor. The history of life seems to be a record of progress. But what does “progress” mean? What does “advanced” mean? The notion of an ordering among organisms dates at least to Aristotle, who arranged living forms on a linear scale based on degree of perfection, also called the scala naturae or Great Chain of Being. Humans are more perfect than other apes, apes more perfect than mice, mice than snakes, snakes than fish, fish than snails, snails than worms, and so on. The Great Chain can also be thought of as a ladder, a ladder of perfection, on which each rung is occupied by some beast, with humans at the top, or in some versions ranking just below angels and God. For more than 2000 years after Aristotle, the Great Chain was understood to be static, meaning that organisms and their rankings did not change over time (Lovejoy 1936). But in the early nineteenth century, Jean-Baptiste Lamarck added an evolutionary component, so that organisms moved up the chain, or progressed, as they evolved. In Lamarck’s view, progress was increasing complexity, with the complexification driven by invisible fluids within organisms (not by adaptation, which he saw as acting mainly to deflect or retard the process; Lamarck 1809).
