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Numerous systems of taxonomy developed in the eighteenth century (that of Linnaeus being the most influential) encouraged naturalists to think about the relationships between the various species and groups of species. And the discovery by paleontologists (mostly amateur) of remains which did not belong to any living species encouraged scientists to consider the possibility that the forms of life changed over time. Pierre Louis Moreau de Maupertuis was probably the first to propose a general theory of evolution, describing a model by which parents passed hereditary material to their offspring.
As with the classification of species, evolutionary discourse was (and still is) implicated in larger questions of politics and religion. Against those who argued for evolution were those who believed that the world had been created perfect by God, and that it therefore admitted no variation and no extinction. The notion of inevitable progression from lower to higher, with the concomitant belief in the extinction of some species, also carried radical political connotations, and as a general rule, French thinkers -- especially those with a liberal political bent, such as Voltaire -- embraced the theory of progressive evolution and rejected the belief in a perfect, immutable world. John Ray was the first to demonstrate convincingly that fossils represent the remains of extinct species, and provided an empirical basis for many progressive political theories.
Among believers, two schools of thought dominated eighteenth-century discussions of evolution: those on one side (such as Bonnet and Cuvier) believed in a continuous and unalterable teleological ascent toward perfection; those on the other (such as Robinet and Lamarck) believed evolution took place through catastrophe and mutation.
Even those who believed in the mutation of species over time did not necessarily accept a strictly evolutionary view. Buffon, for instance, hypothesized that all animals might be descended from one breeding pair, but he held that all genetic mutations introduced ever greater imperfections; animals species, he argued, had devolved from their original state, not evolved.
In the late eighteenth century, some thinkers began to apply theories of evolution not only to the lower animals but to human beings as well. Johann Friedrich Blumenbach carried out important research on the primates and explored their relationship to humans, at once providing further evidence for evolutionists and founding the science of physical anthropology.
Charles Darwin's important contribution was not originating the idea of evolution itself, but of discovering its mechanism, which he termed natural selection. Whereas Lamarck believed that organisms could pass on acquired traits to their offspring, Darwin believed that the random mutation in genetic materials at birth would make some organisms better suited to survive. Those whose random mutations did not prove an advantage would die without passing on the traits to future generations, while those whose mutations gave them an advantage over others would produce offspring with similar traits. He read a paper on the first version of this theory before the Linnaean Society in 1858, and developed at greater length the following year in The Origin of Species.