Early in his career, Lamarck served as tutor to Buffon's son. He was named botanist of the King's Gardens in Paris in 1782, and held the position until the Gardens' closing for the Revolution in 1790. With Buffon's help, in 1794 he was named chair of invertebrate zoology by the National Convention.
He worked in the tradition of Robinet, unlike his student Cuvier, who gave more attention to the work of Bonnet.
Lamarck, studying Linnaeus's system of classification, turned his attention to the lower animals, especially the invertebrates. (In fact the distinction of animals into vertebrates and invertebrates was largely the work of Lamarck.) Whereas Linnaeus had identified only two classes of invertebrates -- insects and worms -- Lamarck distinguished ten (1808). He arranged these classes in order of complexity, and suggested that this is the order through which the species had evolved. He expanded his system from single-celled organisms to man in his Philosophie zoologique (Zoological Philosophy, 1809). There he arranged all animals in a graduated sequence, beginning with mammals and working in order of decreasing complexity to to reptiles, fish, invertebrates, and eventually down to the polyps. This hierarchy represented the sequence of evolution, beginning with the simplest animals and proceeding, through small modifications, to produce all animals. He suggested four laws to explain why and how animal life might change:
Lamarck believed that all species derived their energy from heat and electricity; the more primitive species, he argued, relied on their environment and their behavior was therefore mechanically determined, but the higher animals were able to generate their own heat and electricity, and therefore were able to achieve a measure of self-determination.
Lamarck is best remembered now for Charles Darwin's disagreement with his theory of acquired traits. The most familiar example of this theory is giraffes: Lamarck suggested that giraffes who, through stretching to reach tall trees, make their necks longer, would then pass on longer necks to their offspring.