Final answer:
Lamarck believed that new organs arise in animals through the use and disuse of organs as influenced by the environment, a concept now referred to as Lamarckian inheritance or the inheritance of acquired characteristics.
Step-by-step explanation:
According to Jean-Baptiste Lamarck, new organs arise in animals as a result of the use and disuse of those organs, influenced by the environment or creatures' habits. Lamarck's theory, known as the inheritance of acquired characteristics, posited that if an organism changes during its lifetime in response to its environment, those changes would be inherited by its offspring.
Lamarck famously used the example of the giraffe's long neck to illustrate his theory. He suggested that over generations, giraffes stretched their necks to reach higher leaves for food, which resulted in each subsequent generation having a longer neck. However, this concept of Lamarckian inheritance has been largely discredited in light of modern genetics and the work of Charles Darwin and others in the field of evolutionary biology.