How did Lamarck describe the role of the environment in the evolution of species?

Jean-Baptiste Lamarck described the environment as a primary force that disrupts the natural, progressive complexity of organisms, leading to the diversification of species. According to his theory, changes in the environment create new "needs" (besoins) for an animal.12

To satisfy these needs, animals adopt new behaviors, which result in the increased use or disuse of specific organs. Lamarck proposed that:21
* Use: Continued use of an organ in response to environmental demands causes it to enlarge or strengthen.21
* Disuse: Conversely, the disuse of parts leads to their decline or atrophy.2

Lamarck argued that these material alterations, acquired during an individual's lifetime, were passed on to their offspring. Over many generations, the accumulation of these small, environmentally induced changes resulted in the evolution of species. While Lamarck’s theory was influential in the 19th century, it was later discredited by modern genetics, which has shown that such acquired characteristics are not inherited in the manner he proposed.2134