For much of the eighteenth century, the biological world was seen as a very ordered place. Plants and animals yielded to Linnean classification. Physiological functioning was envisioned in mechanistic terms. And the generation of new animals and plants proceeded from preformed germs that had existed since the creation. All this order arose from God, who had created and organized the world for humans to understand and thereby to appreciate His handiwork and lead moral lives. Even the seemingly disordered, such as monsters and wonders of Nature, were generally brought under the paradigm of order.

All this was to be challenged by mid-century. Mechanistic physiology, based on the analogy of living organisms with machines, was to be considerably broadened by the introduction of Newtonian forces into physiology. The clear borders between the animal and plant kingdoms, and even between the plant and mineral worlds, were to be called into question by new experimental evidence. And the comfortable synthesis of mechanism with reproduction from preexisting germs was to encounter serious opposition from new theories of gradual development that raised the specter of materialism.

Some scholars have characterized eighteenth-century life sciences as a move from mechanism to vitalism, from a view that living phenomena could be explained by matter and motion alone to one that argued that living organisms possess some special force or principle that makes them distinct from dead matter. Yet, as Thomas Hankins has pointed out, creating such absolute dichotomies can be misleading because it ignores the “middle ground.”