One of the first explanations of evolution was put forth by a French biologist, Jean-Baptiste Lamarck. He theorized that animals, in responding to different environments, adopted new
habits. Their new habits caused them to use some organs more and some organs less, which
resulted in the strengthening of the former and the weakening of the latter. New characters
thus acquired by organisms over the course of their lives were passed on to the next
generation. His theory proposed this process of use and disuse, repeated over many
generations, was how species evolved.
He was wrong. Why?

Traits acquired during an organism's lifetime cannot be passed

Explanation:

The law of use and disuse of organs as postulated by Jean Lamarck was wrong due to the fact that environmentally acquired characters cannot be passed to offspring.

Changes to organisms that are not the level of the gene cannot be passed to the offspring. For example, a wrestler that got big muscles due to constant work out cannot pass big muscles to his offspring. The offspring will need to work out in order to have big muscles. This is because big muscles in the wrestler is environmentally acquired.

Only changes that happen to the genome of an organism (mutation) can be passed to offspring.