After years working on plants, Lamarck was appointed curator of invertebrates — another term he coined. He began a series of public lectures. Before 1800, he was an essentialist who believed species were unchanging. After working on the molluscs of the Paris Basin, he grew convinced that transmutation or change in the nature of a species occurred over time. He set out to develop an explanation, which he outlined in his 1809 work, Philosophie Zoologique.

Lamarck developed two laws:

1. In every animal which has not passed the limit of its development, a more frequent and continuous use of any organ gradually strengthens, develops and enlarges that organ, and gives it a power proportional to the length of time it has been so used; while the permanent disuse of any organ imperceptibly weakens and deteriorates it, and progressively diminishes its functional capacity, until it finally disappears.
2. All the acquisitions or losses wrought by nature on individuals, through the influence of the environment in which their race has long been placed, and hence through the influence of the predominant use or permanent disuse of any organ; all these are preserved by reproduction to the new individuals which arise, provided that the acquired modifications are common to both sexes, or at least to the individuals which produce the young.

Lamarck saw spontaneous generation as being ongoing, with the simple organisms thus created being transmuted over time (by his mechanism) becoming more complex and closer to some notional idea of perfection. He thus believed in a teleological (goal-oriented) process where organisms became more perfect as they evolved. During his lifetime he became controversial; his criticism of the palaeontologist Georges Cuvier’s anti-evolutionary stance won him no friends.

Lamarck married three, possibly four, times. His first marriage was to his mistress from 1777, Marie Delaporte, the mother of his first six children, whom he married on her deathbed in 1792. He remarried in 1795 to Charlotte, but she died in 1797. His third wife was Julie Mallet in 1798. She died in 1819. Rumours exist of a fourth wife and widow but no documentary evidence exists of her.

Lamarck died penniless in Paris on 28 December 1829.

Legacy
His defenders believe he is unfairly vilified today. They note that he believed in organic evolution at a time when there was no theoretical framework to explain evolution. He also argued that function precedes form, an issue of some contention among evolutionary theorists at the time. On the other hand, the inheritance of acquired characteristics is now widely rejected.

August Weismann disproved the theory by cutting the tails off mice, demonstrating that the injury was not passed on to the offspring. Jews and other religious groups have been circumcising men for hundreds of generations with no noticeable withering of the foreskin among their descendants. However, Lamarck did not count injury or mutilation as a true acquired characteristic, only those which were initiated by the animal's own needs were deemed to be passed on.

Nowadays, the idea of passing on to offspring characteristics that were acquired during an organism's lifetime is called Lamarckian. This view was, until very recently, thought to be inconsistent with modern genetics, until the discovery of epigenetic inheritance. The memetic theory of cultural evolution could be considered a form of Lamarckian inheritance of non-genetic traits.

Darwin not only praised Lamarck in the third edition of The Origin of Species for supporting the concept of evolution and bringing it to the attention of others, but also accepted the idea of use and disuse, and developed his theory of pangenesis partially to explain its apparent occurrence. Darwin and many contemporaries also believed in the inheritance of acquired characteristics, an idea that was much more plausible before the discovery of the cellular mechanisms for genetic transmission.

The pejorative use of terms like Lamarckian stems from the confusion of students on the mechanisms of evolution. Even when natural selection is explained properly, students tend to think of traits being selected by the organism. It is also sometimes easier to say that "trait X was beneficial so the population got trait X" than to say "trait X was beneficial, individuals without trait X were less likely to propagate, resulting in the population having mostly individuals with trait X." This is a problem related to anthropomorphizing the subject.