Jean-Baptiste Lamarck publishes Philosophie Zoologique

Background. Lamarck had first outlined his newly developing ideas about evolution in a lecture at the Muséum national d'histoire naturelle on 11 May 1800 (21 Floréal, Year VIII). His contribution to evolutionary theory consisted of the first cohesive theory of biological evolution, in which a complexifying force drove organisms up a ladder of complexity, while a second environmental force adapted them to local environments through use and disuse of characteristics, differentiating them from other organisms.

Doctrinal Laws. The book outlined two laws enabling species to acquire characteristics. The first stated that frequent use strengthens organs while disuse causes deterioration. The second law asserted that these acquired modifications are preserved by reproduction, provided they are common to both sexes. This concept is now known as soft inheritance or Lamarckism.

Scientific Reception. Lamarck's views on evolution were dismissed in favour of developments in Darwinism, and his theory achieved fame only after the publication of Charles Darwin's On the Origin of Species in 1859, which spurred critics of Darwin's theory to fall back on Lamarckian evolution as a more well-established alternative. An unnamed assessment holds that the discussion of the species question in the Philosophie Zoologique, whatever might be said for it in 1809, was miserably below the level of the knowledge of half a century later; the same source points to Lyell's trenchant and effectual criticism, published as far back as 1830. Stephen Jay Gould, however, argues that Lamarck was the "primary evolutionary theorist", whose ideas and the structure of his theory set the tone for much subsequent thinking in evolutionary biology.