Although the name "Lamarck" is now associated with a discredited view of evolution, the French biologist's notion that organisms inherit the traits acquired during their parents' lifetime had common sense on its side. In fact, the "inheritance of acquired characters" continued to have supporters well into the 20th century.
Jean Baptiste Lamarck (1744-1829) is one of the best-known early evolutionists. Unlike Darwin, 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. Since simple organisms exist alongside complex "advanced" animals today, Lamarck thought they must be continually created by spontaneous generation.
According to Lamarck, organisms altered their behavior in response to environmental change. Their changed behavior, in turn, modified their organs, and their offspring inherited those "improved" structures. For example, giraffes developed their elongated necks and front legs by generations of browsing on high tree leaves. The exercise of stretching up to the leaves altered the neck and legs, and their offspring inherited these acquired characteristics.
According to Darwin's theory, giraffes that happened to have slightly longer necks and limbs would have a better chance of securing food and thus be able to have more offspring -- the "select" who survive.
Conversely, in Lamarck's view, a structure or organ would shrink or disappear if used less or not at all. Driven by these heritable modifications, all organisms would become adapted to their environments as those environments changed.
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.