If we breed many mice, every mouse that is born will have a tail like its parents. Now, if we surgically remove the tails and then breed the mice once more, we continue to produce mice with tails. This is due to the fact that removing a mouse’s tail does not alter the genes in its reproductive cells (or gametes).
Additionally, since the gene structure of mice is not altered by the acquired property of having severed tails, this trait cannot be handed down to succeeding generations. This leads us to the conclusion that acquired traits—traits that an individual accumulates throughout his or her lifetime—cannot be passed down to offspring and, thus, cannot be the basis for evolution because they are not brought about by a change in DNA.