Lamarck was the first man whose conclusions concerning the origin of species excited much attention. He first did the great service of arousing attention to the probability of all change in the organic, as well as inorganic world, being the result of law and not of miraculous interposition.

His first book was on the Plants of France – Flore Francaise, which brought him acclaim. It remained a standard work for many years. He was elected to the prestigious French Academy of Sciences in preference to scientists with much longer careers behind them. He worked in the Royal Herbarium at the Jardin des Plantes and was instrumental in its reorganization in 1793, into the French Museum of Natural History.

He coined the term invertebrates – (animals without backbones) and categorized the museum’s vast and disorganized collection. His system of dividing and subdividing the organisms according to type, set the standard for later systems of invertebrate taxonomy, and has not yet been discarded.

Discovering that animals varied sometimes by minute degrees, Lamarck began to formulate new ideas about the relationship between animals, and then about the transmutation of species into new ones. His works on invertebrates represent a great advance over existing classifications. He was the first to separate Crustacea, Arachnida and Annelida from the Insecta. His classification of the molluscs was far advanced than anything proposed previously. He broke with tradition in removing the tunicates and the barnacles from the Mollusca.

He also anticipated the work of Schleilden and Schwann in cell theory. He even found time to write on physics and meteorology, including some annual compilations of weather data. But Lamarck is best known for this theory of evolution. Discovering that it is often very difficult to draw a distinct line between two closely related species, Lamarck concluded that species in general, were the result of gradual improvements in organization rather than acts of special creation. The organs of an animal did not give rise to its special habits and faculties, but on the contrary, its habits, mode of life and environment controlled the shape of its body, the number and state of its organs, and lastly, the faculties which it possesses. Thus, characteristics were ‘acquired’. Further, these characteristics were passed on to the offspring's also. Therefore, the theory came to be known as the inheritance of acquired characters.