Unlike Darwin, Lamarck held that evolution was a constant process of striving toward greater complexity and perfection. Even though this belief eventually gave way to Darwin's theory of natural selection acting on random variation, Lamarck is credited with helping put evolution on the map and with acknowledging that the environment plays a role in shaping the species that live in it.
Lamarck believed that living things evolved in a continuously upward direction, from dead matter, through simple to more complex forms, toward human "perfection." Species didn't die out in extinctions, Lamarck claimed. Instead, they changed into other species.