The theory of organ use and disuse was proposed by Jean Baptiste Lamarck.
He established the law of use and disuse, which states that when specific organs become specifically developed as a result of a necessity in the environment, that condition is hereditary and can be passed down to the progeny.
The elongated neck of the giraffe is a classical example used to convey the concept of use and disuse.
According to Lamarck's theory, a giraffe's neck could elongate over a period of time of straining to reach high branches.
This elongation of the neck in giraffes is then passed down from one generation to another.