Inheritance of acquired characteristics
He suggested that traits an organism developed during its life through continual use would grow stronger and be passed on to its descendants. So, a giraffe that continually stretched its neck to eat leaves would have children with longer necks. A deer which developed powerful legs fleeing from predators would have children with powerful legs.
Lamarck's theory was disproven in the 20th century, and is thus another in a long line of discredited evolutionary theories.