This was Lamarck's idea. Here's an example: Suppose giraffes originally had short necks that they stretched to reach high-up leaves in the trees. This continuous stretching of the neck was passed onto offspring, who as a result had slightly longer necks. This continued for multiple generations until we get today's long-necked giraffe. Lamarck was on to something (that something being evolution by natural selection, which Darwin discovered), but his theory wasn't completely correct since organisms can only pass on genes (segments of DNA that code for a characteristic or function) to their offspring. Since "stretching" would not code into DNA, it wouldn't be passed onto offspring, proving Lamarck's theory incorrect.
