In the spirit of Alfred North Whitehead's characterisation of the history of philosophy, Arthur O. Lovejoy sets out a history of the idea of the Great Chain of Being as, essentially, a series of ‘footnotes to Plato’. By this he means, in particular, the predominance of thinking in terms of the principles of plenitude, of continuity, and of unilinear gradation, terms that were not fully formulated by Plato, but that nevertheless came to represent significant cornerstones of the study of natural history in subsequent eras. According to the principle of plenitude,

The principle of continuity, in some ways a development of the idea of plenitude, then comes to dominate natural history as the idea that all living things exist in an ascending series of forms. Finally, the principle of unilinear gradation (through Aristotle), adds to the ideas of the fullness and continuity of beings the sense of their existence and organisation into hierarchies of physical and psychological development.

To preface my discussion of some key debates in nineteenth-century biology, it is worth noting some aspects of eighteenth-century thought that Lovejoy identifies as having a particular effect on this idea of the Chain of Being and its basis in the Platonic principles. Lovejoy notes that, whereas the fields of astronomy, physics and metaphysics had thrown off the yoke of Aristotelianism, biology in the early modern period was characterised by a certain fidelity to Aristotle's categorisation of living things into kinds.