Lamarck's theory on evolution was wrong because organisms pass down traits through predetermined genetic information, not based on environmental adaptations during their lifetime. Lamarck proposed that if an organism used a certain muscle differently, like a giraffe having to consistently reach up higher on a tree for leaves, the offspring would naturally have a longer neck.
Lamarck's theory was based on limited scientific evidence and observation. It was known that animals adapt to their behavior. For example, people that run more than others will become more fit. Lamarck believed that these attributes of fitness were passed down to offspring, but this was disproved with the discovery of genetic information.