Also called the great chain of being and scala natura.
Based on ideas of Plato (c.427-c.347 BC) and Aristotle (384-322 BC), but popularized in biology in the writings of German philosopher Gottfried Leibniz (1646-1716), French naturalist Georges Louis Leclerc, Comte de Buffon (1720-1788), and Swiss philosopher Charles Bonnet (1720-1793).
This is the influential concept that all of nature - from non-living matter to sophisticated organisms to spiritual beings - forms an unbroken physical and metaphysical series.
The theory's biological significance developed in the 18th century, when species were arranged in a graded series or hierarchy.
Certain species, such as the green hydra, were thought to be crucial links in the series. The notion that particular metals, animals or classes of humans outrank others is no longer accepted as part of mainstream science.
Also see: ORTHOGENESIS, speciesism
A Lovejoy, The Great Chain of Being: A Study of the History of an Idea (Cambridge, Mass., 1936)