Philosophie zoologique. 2 vols.

Lamarck was one of the greatest of the comparative anatomists. This work is considered the greatest exposition of his argument that evolution occurred by the inheritance of characteristics acquired by animals as a result of the use or disuse of organs in response to external stimuli. English translation by H. Elliot, 1914. Digital facsimile of the 1809 edition from Google Books at this link.