How did Lamarckism connect an animal’s habits to changes in its organs?

Lamarckism, a theory of evolution proposed by Jean-Baptiste Lamarck, posited that environmental changes create new needs (besoins) in animals. To satisfy these needs, animals adopt new behaviors or habits, which directly influence their physical form.123

According to this theory, the consistent use of a specific organ to meet a new need leads to its strengthening or increase in size, while the disuse of other parts results in their decline or weakening. These material alterations, acquired during an individual's lifetime, were believed to be inherited by the next generation. For example, Lamarck suggested that the giraffe developed a long neck through the persistent habit of stretching to reach high leaves, a trait he argued was passed to its offspring.132

While these ideas were influential in the 19th century, they were later discredited by modern genetics, as there is no evidence that such somatic changes are transmitted to spermatozoa or ova.123