What idea did Lamarck propose that was rejected?

Lamarck proposed the idea of the inheritance of acquired characteristics, suggesting that organisms could pass on traits acquired during their lifetime to their offspring. For example, he believed that if an organism developed a characteristic through use or disuse, such as a giraffe stretching its neck to reach higher leaves, that trait would be inherited by its descendants. This concept was rejected in favor of Darwin's theory of natural selection, which emphasizes genetic variation and heritable traits rather than acquired characteristics. Modern genetics has further supported the idea that acquired traits do not alter an organism's DNA in a way that can be passed to future generations.