How did Lamarckism explain the inheritance of acquired characteristics?

Lamarckism, proposed by Jean-Baptiste Lamarck in 1809, suggests that physical changes acquired by an organism during its lifetime can be passed on to its offspring.1

The theory explains this process through the following mechanism:

• New Needs and Habits: Changes in an environment create new needs (besoins) for an animal, prompting it to adopt new behaviors.23
• Use and Disuse: These new behaviors lead to the increased use of certain organs, causing them to strengthen or enlarge, while the disuse of other parts leads to their decline.23
• Inheritance: Lamarck posited that these resulting material alterations—the acquired characteristics—are transmitted to the next generation, provided both parents underwent the same changes.23

Lamarck famously used the giraffe as an example, suggesting that the species' long neck evolved because generations of individuals stretched to reach higher leaves, a habit that gradually increased the length of their necks over time. While historically influential, this theory is now discredited in modern biology, as there is no evidence that somatic changes are transmitted to germ cells.234